react

React Basics

When to use state

The most important mental model in React to keep in mind is that the view (UI) is a function of the state of a component:

$v = f(s)$ This means that you should declare variables as states using the useState() hook if the value of the variable is used to render the UI. Else, it should not be state.

React rendering pipeline

The react rendering pipeline consists of three components:

1. component code: The code that defines the state and JSX that a component has
2. snapshot: Before any state update takes place that would trigger a re-render,  React creates a snapshot of your component which captures everything React needs to update the view at that particular moment in time. props, state, event handlers, and a description of the UI (based on those props and state) are all captured in this snapshot.
3. view: Based on the snapshot, React takes that description of the UI and uses it to update the View eventually, doing it after batching state calls.

React rerendering

React will only re-render when the state of a component changes. This unintiuitively, however, leads to multiple caveat of React rerendering:

• entire component tree re-renders: If a component has state changes requiring a re-render, then the entire subtree of that component (all subcomponents and JSX) will also re-render, regardless if those sub-components have state changes or not.

When some event handler or effect changes state, React does the re-render pipeline as so:

1. React takes a snapshot of the current values of the props and state inside the component and freezes those for the evaluation of what happens during the event handler.
2. After all state updates in the triggered event-handlers or effects are run, then React takes a new snapshot of all current prop values and state values and compares it to the previous snapshot, and if anything is different (using value equality), then React re-renders. Else it doesn't.

event handlers and effects

State changes can only occur in the context of event handlers or effects, so React treats them specially by taking a snapshot before an event handler or effect that has a state change gets invoked:

NOTE

When an event handler is invoked, that event handler has access to the props and state as they were in the moment in time when the snapshot was created.

Another way you can think about this is whenever an event occurs, regardless of when or how it was triggered, that event will have the same props and state as the snapshot that was created with the render that event is associated with.

if an event handler or effect contains an invocation of useState's updater function and React sees that the new state is different than the state in the snapshot, React will trigger a re-render of the component – creating a new snapshot and updating the view.

Take a look at this example:

import * as React from "react"

export default function VibeCheck () {
  const [status, setStatus] = React.useState("clean")

  const handleClick = () => {
    setStatus("dirty")
    alert(status)
  }

  return (
    <button onClick={handleClick}>
      {status}
    </button>
  )
}

What gets alerted is the string "clean" due to this line of reasoning:

1. Event handlers that contain state updates use variables in the snapshot captured by react right before the invocation of the event handler.
2. In the snapshot, the status state has a value of "clean", thus even though we do the state update with setStatus("dirty"), the state update doesn't happen until all event handler and effects scheduled to run have finished. Thus what gets alerted is "clean"

Even if an event handler has a state update, the component will only re-render if the state update changes the state to a value that is unequal (value checking) to the snapshot state value.

In the below example, even though the handleClick() event handler has a state update, because the updated state doesn't differ between snapshots, the component does not re-render.

import * as React from "react"

export default function Counter () {
  console.count("Rendering Counter")
  const [count, setCount] = React.useState(0)

  const handleClick = () => {
    console.count("click")
    setCount(count)
  }

  return (
    <button onClick={handleClick}>
      🤨
    </button>
  )
}

IMPORTANT

Again, React will only re-render if the event handler contains an invocation of useState's updater function (✅) and React sees that the new state is different than the state in the snapshot (❌).

The new snapshot is taken AFTER all state update functions in the invoked event handlers or effects have run. This prevents unnecessary re-renders after each state change, where React lazily takes the snapshot after all state-update functions have run, and uses only the last snapshot to update the UI.

This leads into an algorithm called batching:

batching

NOTE

Basically, after a bunch of setState() calls, there is only one re-render with all new updated state values after all effects and event handler executions have completed.

So in an event handler or effect, if multiple setState() calls for a specific state are invoked, then only the last set state call will be set as the updated state. This is called batching.

React will only re-render once per event handler invocation or effect change, after all state updates are completed.

Whenever React encounters multiple invocations of the same updater function (e.g. setCount in our example), it will keep track of each of them, but only the result of the last invocation will be used as the new state.

const handleClick = () => {
  // queued snapshot: count = 1
  setCount(1)
  // queued snapshot: count = 2
  setCount(2)
  // queued snapshot: count = 3
  setCount(3)
  
  // final snapshot: count = 3
}

So in this example, the new state will of course be 3.

overriding batching

there is a way to tell React to use the value of the previous invocation of the updater function instead of replacing it. To do that, you pass the updater function a function itself that will take in the value from the most recent invocation as its argument.

const handleClick = () => {
  // queued snapshot: count = 1
  setCount(1)
  // queued snapshot: count = 2
  setCount(2)
  // queued snapshot: count = count + 3 -> count = 5
  setCount((c) => c + 3)
  
  // final snapshot: count = 5
}

Memoization

If a component has state changes requiring a re-render, then the entire subtree of that component (all subcomponents and JSX) will also re-render, regardless if those sub-components have state changes or not.

A React performance enhancement to mitigate this excessive re-rendering of child components is called memoization, where we memoize components by wrapping them in the memo() function from React.

Any memoized child component will only re-render during a parent's re-render cycle if the props passed to that child component change (or state in the child component changes), where props are compared via primitive value equality.

Strict Mode

React strict mode is used for development use cases where by wrapping your entire app in the <React.StrictMode> component, you enable strict mode, where every component will re-render at least twice regardless of what memoization tactics you use.

TIP

This is helpful in unveiling common bugs.

In production, strict mode is disabled.

Referential equality

• primitive values: their in-memory values are the variable value themselves, liek a string, boolean, int, etc.
• reference values: their in-memory values are their memory addresses, not the values.

This means value equality checking is intuitive and works well for primitive values, but only works for reference values when two variables share the exact same reference.

React uses primitive values to check if props change, state changes, and if dependencies have changed, so it's important to cache object references to avoid constant re-renders by constantly creating new objects (which have different memory addresses).

• useCallback(func, deps): returns a cached version of the function which is only recreated on re-renders where one of the dependencies in its dependency array changes.
• useMemo(func, deps): invokes the function and gets what it returns, and it value is only recreated on re-renders where one of the dependencies in its dependency array changes.

This is the correct way to prevent re-renders of memoized components but have object props:

import * as React from "react"
import Wave from "./Wave"

function Greeting ({ name }) {
  const [index, setIndex] = React.useState(0)
  const [waveIndex, setWaveIndex] = React.useState(0)

  const greetings = ['Hello', "Hola", "Bonjour"]

  const handleClick = () => {
    const nextIndex = index === greetings.length - 1
      ? 0
      : index + 1
    setIndex(nextIndex)
  }

  const handleWaveClick = React.useCallback(() => {
    setWaveIndex((i) => {
      return i === 5 ? 0 : i + 1
    })
  }, [])

  const options = React.useMemo(() => {
    return {
      animate: true,
      tone: waveIndex
    }
  }, [waveIndex])

  return (
    <main>
      <h1>{greetings[index]}, {name}</h1>
      <button onClick={handleClick}>
        Next Greeting
      </button>
      <Wave onClick={handleWaveClick} options={options} />
    </main>
  )
}

export default function App () {
  return <Greeting name="Tyler" />
}

Effects in React

Rules of side effects in components

1. components must be pure: To keep the component function pure, no side effects can be executed within the main body of the component. When a component renders, it should do so without running into any side effects
2. side effects triggered by an event should be put in that event handler
3. If a side effect is synchronizing your component with some external system, put that side effect in a useEffect() hook.

NOTE

Basically, the main rule of having side effects in components is that they should be encapsulated into code that doesn't deal with render cycle, like in useEffect blocks and event handlers.

No, this state hack doesn't work

Take a look at this example below. You might think that it's fine to use a side-effect like fetching from local storage if it's for initializing state, but it's still a side effect and thus will rerun on every re-render of the component, not behaving as planned.

• what you think should happen: the index state gets initialized with the locally stored value
• what actually happens: on every re-render, the side effect runs again, thus the state gets reinitialized every time with the locally stored value, even if that's not what you want.

import * as React from "react"

function Greeting ({ name }) {
  const [index, setIndex] = React.useState(
    Number(localStorage.getItem("index"))
  )

  const greetings = ['Hello', "Hola", "Bonjour"]

  const handleClick = () => {
    const nextIndex = index === greetings.length - 1
      ? 0
      : index + 1
    setIndex(nextIndex)

    localStorage.setItem("index", nextIndex)
  }

  return (
    <main>
      <h1>{greetings[index]}, {name}</h1>
      <button onClick={handleClick}>Next Greeting</button>
    </main>
  )
}

What's the solution for initializing state with local storage? Put that side effect in a useEffect() with an empty dependency array so it only runs once.

useEffect

The useEffect() hook can contain side-effects and state update invocations, all of which run after every render or re-render.

NOTE

when to use useEffect()

---

If a side effect is synchronizing your component with some external system, put that side effect inside useEffect. Here are some use cases:

• Syncing with local storage
• Using fetch() API or fetching external data
• Querying DOM data

However, to prevent unnecessary re-renders, we make use of the dependency array to let React know which variables the side effect code in our useEffect() invocation is dependent on, and to only run the side effect when those variables change (value equality or reference equality).

useEffect lifecycle

useEffect works by removing the side effect from React's rendering flow and delaying its execution until after the component has rendered.

Here is the simplified lifecycle.

1. React re-renders
2. All useEffect blocks will run, executing any side-effects

However, if you have a dependency array, then this changes and the useEffect block will only run after a re-render if one of its dependencies have changed.

NOTE

Another way to think about it

---

The whole goal of useEffect is to synchronize your component with some external system. Whenever any of the dependencies that the effect needs in order to synchronize change, React, should resynchronize.

useEffect antipattern and reactive values

useEffect blocks should NOT be used for reacting to changes in values. That's what event handlers are for. Rather, you should only use useEffect blocks for synchronizing UI and state to external systems.

Reactive values are any values that change between re-renders, for example, like state.

WARNING

Having reactive values in your dependency array is a red flag that you might be in the anti-pattern of reacting to state changes, but it's fine to have reactive values in the dependency array if you are using that value to sync to an external system, like the network or local storage.

NOTE

A reactive value is any value that can change between re-renders. Props, state, or any variables defined inside of a component are all reactive values.

useEffect cleanup function

The cleanup function in a useEffect block runs in two scenarios:

• Scenario 1 (component unmounts): When the component unmounts, the cleanup function will run
• Scenario 2 (effect is scheduled to rerun): If any of the values in the dependency array changes, then the effect is scheduled to rerun, and thus the cleanup function is called before the rerun of that effect.

NOTE

If you return a function from your effect, React will call that function each time before it ever calls your effect again, and then one final time when the component is removed from the DOM.

Here is the general lifecycle of rerendering and running effects:

1. React renders for first time
2. If scheduled to run, useEffect block runs
3. On all subsequent re-renders: if useEffect is scheduled to run again, then cleanup function executes first with previous snapshot, then the effect runs.

Complete Data Fetching Example

First, notice that our dependency array only includes one element, id. Again, this isn't telling React to re-run the effect when id changes (though, that's what happens). Instead, we're giving React an array of all of the dependencies our effect needs to re-synchronize with the outside system. As a byproduct of that, whenever React sees that id has changed between renders, it will re-run the effect.

import React, { useState } from 'react'

async function fetchPokemon(id) {
  const abortController = new AbortController()
  try {
    const res = await fetch(
      `https://pokeapi.co/api/v2/pokemon/${id}`,
      {
        signal: abortController.signal
      }
    )

    if (res.ok === true) {
      return {
        error: null,
        response: await res.json(),
        abortController
      }
    }

    throw new Error(`Error fetching pokemon #${id}`)
  } catch (e) {
    return {
      error: e,
      response: null,
      abortController
    }
  }
}

// export default function MyApp() {
//   return <div>Hello World</div>;
// }


// Main App
export default function App() {
  const [id, setId] = React.useState(1)
  const [pokemon, setPokemon] = React.useState(null)
  const [loading, setLoading] = React.useState(true)
  const [error, setError] = React.useState(null)

  React.useEffect(() => {
    const handleFetchPokemon = async () => {
      setLoading(true)
      setError(null)

      const { error, response } = await fetchPokemon(id)

      if (error) {
        setError(error.message)
      } else {
        setPokemon(response)
      }

      setLoading(false)
    }

    handleFetchPokemon()
  }, [id])

  if (loading) {
    return <p>loading...</p>
  }

  if (error) {
    return <p>error {error}</p>
  }

  if (!pokemon && !loading) {
    return "no pokemon"
  }

  return (
    <main>
      {JSON.stringify({ id, pokemon }, null, 2)}
    </main>
  )
}

However, there is still a main problem we have to deal with.

Whenever we call fetchPokemon, because it's an asynchronous request, we have no idea how long that request will take to resolve. It's completely possible that, while we're in the process of waiting for a response, our Carousel updates id, which causes a re-render, which causes our effect to run again with a different id.

In this scenario, we now have two requests in flight, both with different ids. Worse, we have no way of knowing which one will resolve first. In both scenarios, we're calling setPokemon when the request resolves. That means, because we don't know in which order they'll resolve, pokemon, and therefor our UI, will eventually be whatever request was resolved last. To make it worse, you'll also get a flash of the Pokémon that resolves first, before the second one does.

To solve this, we will use a cleanup function in the effect which will cleanup the stale fetch requests in the previous snapshot by aborting them.

Notice that the cleanup function is only called for ids that are no longer relevant. This makes sense because the cleanup function for the most recent effect won't be called until either another effect runs (making it stale) or the component has been removed from the DOM (irrelevant in this scenario).

import React, { useState } from 'react'
import Carousel from "./Carousel"
import PokemonCard from "./PokemonCard"

function fetchPokemon(id) {
  const abortController = new AbortController()
    const res = fetch(
      `https://pokeapi.co/api/v2/pokemon/${id}`,
      {
        signal: abortController.signal
      }
    )

    return {
      response: res,
      abortController
    }
}


// Main App
export default function App() {
  const [id, setId] = React.useState(1)
  const [pokemon, setPokemon] = React.useState(null)
  const [loading, setLoading] = React.useState(true)
  const [error, setError] = React.useState(null)

   const handlePrevious = () => {
    if (id > 1) {
      setId(Math.max(id - 1, 1)) 
    }
  }

  const handleNext = () => setId(id + 1)

  React.useEffect(() => {
  // 1. Create the controller instance *inside* the effect
  const controller = new AbortController();
  
  const handleFetchPokemon = async () => {
    setLoading(true);
    setError(null);

    try {
      // Pass the signal directly to the fetch request
      const res = await fetch(
        `https://pokeapi.co/api/v2/pokemon/${id}`,
        { signal: controller.signal }
      );

      if (!res.ok) throw new Error(`Error fetching pokemon #${id}`);
      
      const data = await res.json();
      setPokemon(data);
    } catch (e) {
      // 3. When a fetch is aborted, it throws an 'AbortError'. 
      // We ignore it so we don't flash error states for stale requests.
      if (e.name !== 'AbortError') {
        console.error(e);
        setError(e);
        setPokemon(null);
      }
    } finally {
      // Only turn off loading if this request wasn't aborted
      if (!controller.signal.aborted) {
        setLoading(false);
      }
    }
  };

  handleFetchPokemon();

  // 2. The Cleanup Function: Runs when 'id' changes or component unmounts
  return () => {
    controller.abort();
  };
}, [id]);

  if (loading) {
    return <p>loading...</p>
  }

  if (error) {
    return <p>error</p>
  }

  if (!pokemon && !loading) {
    return "no pokemon"
  }

  return (
    <Carousel onPrevious={handlePrevious} onNext={handleNext}>
      {JSON.stringify({
        name: pokemon.name,
        id
      }, null, 2)}
    </Carousel>
  )
}

Refs in React

Sometimes you need to preserve a variable value across renders but you don't use that variable to render the UI. Making this value a state would be an antipattern, as React re-renders when state gets updated.

The useRef hook creates a value that is preserved across renders but won't trigger a re-render when it changes. Here are some use cases:

• setInterval() or setTimeout() Id: It's important to keep track of the id so you can cancel intervals or timeouts, but you never actually directly render the id in the UI, so it should NOT be state. This is the perfect use case for a Ref.
• DOM element: If you want to keep track of some DOM element across re-renders, you should use refs rather than state because you never directly render this DOM element, so it should not be state, otherwise it would cause unnecessary re-renders.
• debouncing and throttling: stores debouncing timers in a ref to avoid unnecessary rerenders

ref vs state example

Here is an example where we incorrectly use state when we should be using a ref:

• main issue: The id state is not rendered anywhere in the view, so invoking setId causes unnecessary re-renders and is thus an antipattern.

import * as React from "react"
import { formatTime } from "./utils"

export default function Stopwatch () {
  const [seconds, setSeconds] = React.useState(0)
  const [running, setRunning] = React.useState(false)
  const [id, setId] = React.useState(null)

  const handleClick = () => {
    if (running === false) {
      const id = window.setInterval(() => {
        setSeconds(s => s + 1) 
      }, 1000)
      setId(id)
      setRunning(true)
    } else {
      window.clearInterval(id)
      setId(null)
      setRunning(false)
    }
  }

  return (
    <main>
      <h1>{formatTime(seconds)}</h1>
      <button
        style={{background: running === true ? 'var(--red)' : 'var(--green)'}}
        onClick={handleClick}>
          {running === true ? 'Stop' : 'Start'}
      </button>
    </main>
  )
}

This is the improved version of this example, where we use a ref for the id state as to preserve the variable across re-renders but not use it for the view:

import * as React from "react"
import { formatTime } from "./utils"

export default function Stopwatch () {
  const [seconds, setSeconds] = React.useState(0)
  const [running, setRunning] = React.useState(false)
  const ref = React.useRef(null)

  const handleClick = () => {
    if (running === false) {
      ref.current = window.setInterval(() => {
        setSeconds(s => s + 1) 
      }, 1000)
      setRunning(true)
    } else {
      window.clearInterval(ref.current)
      setRunning(false)
    }
  }

  return (
    <main>
      <h1>{formatTime(seconds)}</h1>
      <button
        style={{background: running === true ? 'var(--red)' : 'var(--green)'}}
        onClick={handleClick}>
          {running === true ? 'Stop' : 'Start'}
      </button>
    </main>
  )
}

TIP

Basically use useState for a persistent variable whenever you reference it anywhere in the JSX (even if you don't directly render it). Else use useRef, if it's nowhere in the JSX return rendering portion.

ref vs effects summary

• When to use useEffect: If you have a side effect that is triggered by an event, put it in an event handler. If you have a side effect that is synchronizing your component with some outside system, put it inside of useEffect.
• When to use useRef: If you need to preserve a value across renders, but that value has nothing to do with the view, and therefore, React doesn't need to re-render when it changes, put it in a ref using useRef.

Refs with DOM elements

By putting the ref= prop on a JSX element or React component and passing a ref variable, you can control that JSX element or component via DOM APIs through the ref variable.

import * as React from "react"
import Toolbar from "./Toolbar"

export default function App() {
  const white = React.useRef(null)
  const black = React.useRef(null)
  const yellow = React.useRef(null)

  const handleClick = (type) => {
    let ref = null

    if (type === "white") ref = white
    if (type === "yellow") ref = yellow
    if (type === "black") ref = black

    ref.current.scrollIntoView({
      behavior: "smooth",
      block: "nearest",
    })
  }

  return (
    <div>
      <Toolbar handleClick={handleClick} />
      <div ref={white} className="white" />
      <div ref={yellow} className="yellow" />
      <div ref={black} className="black" />
    </div>
  )
}

Here is a ref example with multiple moving parts, where the main goal is to provide a dynamic debounce timing where the user cannot switch the toggle while the element is animating.

• active state: updates the toggle switch, thus should be state since it renders to the view.
• node ref: attaches to the animating element so we can hook into its animations and wait for them to finish.
• animating ref: persistent boolean flag used to mark whether the element is currently animating or not. If the element is not currently animating (animating.current === false) then we are allowed to change the toggle.

import * as React from "react"
import Diagram from "./Diagram"
import ReactLogo from "./ReactLogo"

export default function App() {
  const [active, setActive] = React.useState(false)
  const node = React.useRef(null)
  const animating = React.useRef(false)

  React.useEffect(() => {
    const checkAnimations = async () => {
	  // mark animating as true
      animating.current = true;
      
      // await all animations of node.current to finish
      const animations = node.current.getAnimations({ subtree: true })
      const promises = animations.map((animation) => animation.finished)
      await Promise.all(promises)
      
      // mark animating as false
      animating.current = false;
    }

    checkAnimations()
  })

  const handleToggle = () => {
	// if element is not animating, then user can change toggle state.
    if (animating.current === false) {
      setActive(!active)
    }
  }

  return (
    <div
      ref={node}
      className={`diagram ${active ? "react-approach" : "trad-approach"}`}
    >
      <Diagram />
      <div className="toggle-diagram">
        <input
          id="toggle"
          className="toggle-input"
          type="checkbox"
          checked={active}
          onChange={handleToggle}
        />
        <label htmlFor="toggle" className="toggle-label">
          <div
            className="toggle-options"
            data-checked="React"
            data-unchecked="Traditional"
          >
            <div className="toggle-switch">
              <span className="toggle-marker">
                <ReactLogo />
              </span>
            </div>
          </div>
        </label>
      </div>
    </div>
  )
}

React context

When to use context

IMPORTANT

This is a huge misconception when it comes to Context. Whether what you're teleporting is a static object, function, state, or anything else – it doesn't change that fact that Context is a teleporter, not a manager of state.

Re-rendering data in context follows the common react model of $v=f(s)$, where if state in a context changes, then the entire component tree of the context provider and what ever subcomponents are nested inside it gets re-rendered.

This means it's extremely important to use context only when you don't have a lot of re-renders, otherwise you will have to constantly repaint your app. Because context providers often wrap your whole app, when the context provider must re-render due to a state change in the context, that re-renders the entire app.

To avoid a shit ton of re-renders when using context, here are some tips to keep in mind:

1. pick the right tool for the job: only use react context when it makes sense to do so, like when any state in the context does not change often.
2. use useRef for persistent context values when it makes sense: If you need a persistent variable to export in the context but it's not used anywhere in your app to update the view, make it a ref instead of state as to avoid unnecessary re-renders.

useReducer

Intro

The reducer pattern is a functional programming pattern that takes a collection as input and returns a single value as output. The way you get to that single value is by invoking a reducer function for every element in the collection.

What if instead of our collection being an array, it was a collection of user actions that happened over time? Then, whenever a new user action occurred, we could invoke our reducer to get the new state of our application.

How to use useReducer

Because it's such a helpful pattern, React comes with a built-in Hook called useReducer that functionally behaves like useState, but allows you to manage your state using the reducer pattern.

The API for useReducer is similar to what we saw earlier with Array.reduce, but with one big difference. Instead of just returning the state, similar to useState, useReducer also returns a way to update that state.

What's different is instead of returning an updater function, it returns a function called dispatch that when called, will invoke the reducer function.

import * as React from "react"

function reducer(state, value) {
  const nextState = state + value

  console.log(
    `Reducer invoked. state: ${state}, value: ${value}, nextState: ${nextState}`
  )

  return nextState
}

const initialState = 0

export default function Counter() {
  const [count, dispatch] = React.useReducer(
    reducer, initialState
  )

  const handleIncrement = () => {
    dispatch(1) // invokes reducer with state, value=1
  }

  return (
    <main>
      <h1>{count}</h1>
      <button onClick={handleIncrement}>+</button>
    </main>
  )
}

useReducer vs useState

Here is when to use useReducer vs useState:

• use useState when If different pieces of state update independently from one another
• use useReducer when if your state tends to be updated together or if updating one piece of state is based on another piece of state

When using a reducer, we decouple how the state updates from the action that triggered the update. This makes using useReducer preferable to using useState in the following scenarios:

• minimizing event listeners and interval teardown and setup: in a useEffect where we want to setup event listeners or intervals and we depend on a piece of state
  • if we use useState, then we're recreating and tearing down those event listeners and intervals on every state change.
  • If we use useReducer, then we don't have dependecy on the state, instead we just dispatch actions, which means we can omit state from the dependency array and thus run the useEffect block less.
• updating state where state updates are tightly coupled
• Form state: in forms, you may need lots of state variables, and clearing and submitting forms require an imperative way of state updates, while useReducer updates state in a declarative way.

use case 1: tightly coupled state updates

Here is an example where state updates rely on other pieces of state, so it's better to use useReducer rather than useState:

import * as React from "react"
import Slider from "./Slider"

function reducer(state, action) {
  if (action.type === "increment") {
    return {
      count: state.count + state.step,
      step: state.step,
    }
  } else if (action.type === "decrement") {
    return {
      count: state.count - state.step,
      step: state.step,
    }
  } else if (action.type === "reset") {
    return {
      count: 0,
      step: state.step,
    }
  } else if (action.type === "updateStep") {
    return {
      count: state.count,
      step: action.step,
    }
  } else {
    throw new Error("This action type isn't supported.")
  }
}

export default function Counter() {
  const [state, dispatch] = React.useReducer(reducer, {
    count: 0,
    step: 1
  })

  const handleIncrement = () => dispatch({type: "increment"})
  const handleDecrement = () => dispatch({type: "decrement"})
  const handleReset = () => dispatch({type: "reset"})
  const handleUpdateStep = (step) => dispatch({type: "updateStep", step})

  return (
    <main>
      <h1>{state.count}</h1>
      <div>
        <button onClick={handleDecrement}>-</button>
        <button onClick={handleIncrement}>+</button>
        <button onClick={handleReset}>0</button>
      </div>
      <Slider 
        min={1}
        max={10}
        onChange={handleUpdateStep} 
      />
    </main>
  )
}

With that, we see another subtle but powerful benefit of useReducer you might have missed. Because the reducer function is passed the current state as the first argument, it's simple to update one piece of state based on another piece of state. In our example, we can see this in how we're updating count based on the value of step.

TIP

In fact, I'd go as far as to say whenever updating one piece of state depends on the value of another piece of state, reach for useReducer.

use case 2: imperative vs declarative state updates

The reason useReducer can be more declarative is because it allows us to map actions to state transitions. This means, instead of having a collection of setX invocations, we can simply dispatch the action type that occurred. Then our reducer can encapsulate the imperative, instructional like code.

const handleSubmit = async (e) => {
  // e.preventDefault()
  // setSubmitting(true)
  // setError(null)
  // setSuccess(false)

  // try {
  //   await subscribe({ name, email, marketing })
  //   setSubmitting(false)
  //   setName("")
  //   setEmail("")
  //   setMarketing(true)
  //   setSuccess(true)
  // } catch (e) {
  //   setSubmitting(false)
  //   setSuccess(false)
  //   setError(e.message)
  // }
  
  e.preventDefault()
  dispatch({ type: "submit" })

  try {
    await subscribe({ name, email, marketing })
    dispatch({ type: "success" })
  } catch (e) {
    dispatch({ type: "error", error: e.message })
  }
}

Notice that we're describing what we want to do - submit. Then, based on that result, success or error. That's a lot cleaner and easier to reason about than our imperative solution.

import * as React from "react";

const initialFormData = {
  name: "",
  email: "",
  address: "",
  city: "",
  zipcode: ""
};

const initialState = {
  currentStep: 1,
  formData: initialFormData
}

function reducer(state, action) {
  
  if (action.type === "next") {
    return {
      ...state,
      currentStep: state.currentStep + 1
    }
  }
  else if (action.type === "previous") {
    return {
      ...state,
      currentStep: state.currentStep - 1
    }
  }

  else if (action.type === "change") {
    if (!action.payload) throw new Error("empty payload for 'change' action")
    const {name, value} = action.payload
    return {
      formData: {
        ...state.formData,
        [name]: value
      },
      currentStep: state.currentStep
    }
  }

  else if (action.type === "submit") {
    return initialState
  }

  else {
    throw new Error("unhandled action type " + action.type)
  }
  
}


export default function MultistepFormReducer() {
  // const [currentStep, setCurrentStep] = React.useState(1);
  // const [formData, setFormData] = React.useState(initialFormData);
  const [state, dispatch] = React.useReducer(reducer, initialState)
  const {currentStep, formData} = state

  const handleNextStep = () => {
    // setCurrentStep(currentStep + 1);
    dispatch({type: "next"})
  };

  const handlePrevStep = () => {
    dispatch({type: "previous"})
    
  };

  const handleChange = (e) => {
    dispatch({
      type: "change",
      payload: {
        name: e.target.name,
        value: e.target.value
      }
    })
    // setFormData({ ...formData, [e.target.name]: e.target.value });
  };

  const handleSubmit = (e) => {
    e.preventDefault();
    alert("Thank you for your submission");
    dispatch({type: "submit"})
    // setCurrentStep(1);
    // setFormData(initialFormData);
  };

  if (currentStep === 1) {
    return (
      <form onSubmit={handleSubmit}>
        <h2>Personal Information</h2>
        <div>
          <label>Step {currentStep} of 3</label>
          <progress value={currentStep} max={3} />
        </div>
        <div>
          <label htmlFor="name">Name</label>
          <input
            required
            name="name"
            id="name"
            placeholder="Enter your name"
            value={formData.name}
            onChange={handleChange}
          />
        </div>
        <div>
          <label htmlFor="email">Email</label>
          <input
            required
            name="email"
            id="email"
            type="email"
            placeholder="Enter your email"
            value={formData.email}
            onChange={handleChange}
          />
        </div>
        <button type="button" className="secondary" onClick={handleNextStep}>
          Next
        </button>
      </form>
    );
  } else if (currentStep === 2) {
    return (
      <form onSubmit={handleSubmit}>
        <h2>Address</h2>
        <div>
          <label>Step {currentStep} of 3</label>
          <progress value={currentStep} max={3} />
        </div>
        <div>
          <label htmlFor="address">Address</label>
          <input
            required
            name="address"
            id="address"
            type="address"
            placeholder="What is your address?"
            value={formData.address}
            onChange={handleChange}
          />
        </div>
        <div>
          <label htmlFor="city">City</label>
          <input
            required
            name="city"
            id="city"
            placeholder="What city do you live in?"
            value={formData.city}
            onChange={handleChange}
          />
        </div>
        <div>
          <label htmlFor="zipcode">Zipcode</label>
          <input
            required
            name="zipcode"
            id="zipcode"
            type="number"
            placeholder="What is your zipcode?"
            value={formData.zipcode}
            onChange={handleChange}
          />
        </div>
        <div>
          <button className="secondary" type="button" onClick={handleNextStep}>
            Next
          </button>
          <button type="button" className="link" onClick={handlePrevStep}>
            Previous
          </button>
        </div>
      </form>
    );
  } else if (currentStep === 3) {
    return (
      <form onSubmit={handleSubmit}>
        <h2>Confirm your information:</h2>
        <div>
          <label>Step {currentStep} of 3</label>
          <progress value={currentStep} max={3} />
        </div>
        <table>
          <tbody>
            {Object.keys(formData).map((key) => {
              return (
                <tr key={key}>
                  <td>{key}</td>
                  <td>{formData[key]}</td>
                </tr>
              );
            })}
          </tbody>
        </table>
        <div>
          <button className="primary" type="submit">
            Submit
          </button>
          <button type="button" className="link" onClick={handlePrevStep}>
            Previous
          </button>
        </div>
      </form>
    );
  } else {
    return null;
  }
}

use case 3: decoupling state updates from state values, leading to less setup and tear down

This is the way where if we use state, then we create an interval and tear it down each time step changes, which is not ideal:

import * as React from "react"
import Slider from "./Slider"

export default function Counter() {
  const [count, setCount] = React.useState(0)
  const [step, setStep] = React.useState(1)

  const handleIncrement = () => setCount(count + step)
  const handleDecrement = () => setCount(count - step)
  const handleReset = () => setCount(0)
  const handleUpdateStep = (step) => setStep(step)

  React.useEffect(() => {
    console.log('useEffect called')
    const id = window.setInterval(() => {
      setCount((c) => c + step)
    }, 1000)

    return () => window.clearInterval(id)
  }, [step])

  return (
    <main>
      <h1>{count}</h1>
      <div>
        <button onClick={handleDecrement}>-</button>
        <button onClick={handleIncrement}>+</button>
        <button onClick={handleReset}>0</button>
      </div>
      <Slider 
        min={1}
        max={10}
        onChange={handleUpdateStep} 
      />
    </main>
  )
}

But if we use useReducer, we solve this issue since we decouple state updates from their current values, thus meaning we can have an empty dependency array:

import * as React from "react"
import Slider from "./Slider"

function reducer(state, action) {
  if (action.type === "increment") {
    return {
      count: state.count + state.step,
      step: state.step,
    }
  } else if (action.type === "decrement") {
    return {
      count: state.count - state.step,
      step: state.step,
    }
  } else if (action.type === "reset") {
    return {
      count: 0,
      step: state.step,
    }
  } else if (action.type === "updateStep") {
    return {
      count: state.count,
      step: action.step,
    }
  } else {
    throw new Error("This action type isn't supported.")
  }
}

export default function Counter() {
  const [state, dispatch] = React.useReducer(reducer, {
    count: 0,
    step: 1
  })

  const handleIncrement = () => dispatch({ type: "increment" })
  const handleDecrement = () => dispatch({ type: "decrement" })
  const handleReset = () => dispatch({ type: "reset" })
  const handleUpdateStep = (step) => dispatch({ type: "updateStep", step })

  React.useEffect(() => {
    console.log("useEffect called")
    const id = window.setInterval(() => {
      dispatch({ type: "increment" })
    }, 1000)

    return () => window.clearInterval(id)
  }, [])

  return (
    <main>
      <h1>{state.count}</h1>
      <div>
        <button onClick={handleDecrement}>-</button>
        <button onClick={handleIncrement}>+</button>
        <button onClick={handleReset}>0</button>
      </div>
      <Slider 
        min={1}
        max={10}
        onChange={handleUpdateStep} 
      />
    </main>
  )
}

NOTE

useReducer also offers a bit more flexibility than useState since it allows you to decouple how the state is updated from the action that triggered the update - typically leading to more declarative state updates.

useLayoutEffect

The useLayoutEffect runs side effects the same way as useEffect except that the side effects (including state changes) runs synchronously BEFORE the browser gets painted.

Here is the different between useLayoutEffect and useEffect:

• useLayoutEffect: runs synchronously after the rendering stage but before browser paint
• useEffect: runs asynchronously after the rendering stage and after browser paint

React guarantees that the code inside useLayoutEffect and any state updates scheduled inside it will be processed before the browser repaints the screen. This lets your component use layout information for rendering – as we're doing in our example with the browser's dimensions.

NOTE

Use useLayoutEffect if you need to synchronize state with the DOM or layout, basically for some use case where the side effect needs to run before the browser paints the screen.

WARNING

Don't use useLayoutEffect unless you're synchronizing externally with the layout. The only thing to keep in mind with useLayoutEffect is you don't want to use it unnecessarily. If you're not synchronizing layout information with your component, you don't need it. Unless you absolutely have to, intentionally blocking React from updating the UI is not a great idea.

useSyncExternalStore

Use the useSyncExternalStore hook to subscribe to an external store or variable that can change values throughout the lifetime of the app, like navigator.userAgent or navigator.onLine.

This is an example where useState + useEffect is subpar, and useSyncExternalStore would work much better

import * as React from "react"

export default function App () {
  const [networkStatus, setNetworkStatus] = React.useState("online")

  React.useEffect(() => {
    const handleChange = () => {
      setNetworkStatus(navigator.onLine ? "online" : "offline")
    }

    window.addEventListener("online", handleChange)
    window.addEventListener("offline", handleChange)

    return () => {
      window.removeEventListener("online", handleChange)
      window.removeEventListener("offline", handleChange)
    }
  }, [])

  return (
    <div>
      <span className={networkStatus} />
      <label>{networkStatus}</label>
    </div>
  )
}

In scenarios where a piece of non-React state is already being managed by some outside system –  whether that's the browser, another library, or even your own custom data store – you can use React's built-in useSyncExternalStore hook to subscribe a component to that state without needing to redundantly duplicate it as React state in your component.

This hook requires two arguments:

• getSnapshot(): a function that returns the external state
• subscribe(): a subscription function that returns a callback function that gets invoked when removing the subscription.

This hook returns whatever getSnapshot() returns as a state variable, but the subscribe() function determines the subscription and unsubscription process and manages when to call getSnapshot() behind the scenes, updating the state automatically through the subscription.

const getSnapshot = () => {
	return snapshot
}

const subscribe = () => {
	// subscription code here
}

const snapshot = React.useSyncExternalStore(subscribe, getSnapshot)

This is the complete example:

import * as React from "react"

const getSnapshot = () => {
  return navigator.onLine ? "online" : "offline"
}

const subscribe = (callback) => {
  window.addEventListener("online", callback)
  window.addEventListener("offline", callback)

  return () => {
    window.removeEventListener("online", callback)
    window.removeEventListener("offline", callback)
  }
}

export default function App () {
  const networkStatus = React.useSyncExternalStore(
    subscribe, 
    getSnapshot
  )

  return (
    <div>
      <span className={networkStatus} />
      <label>{networkStatus}</label>
    </div>
  )
}

Performance

The way useSyncExternalStore decides if it should resubscribe to the store or re-render the UI is through value equality: `

• resubscription logic: If the subscribe function you pass in has a different reference value than the previous snapshot, it runs the subscription cleanup function and re-subscribes.
• re-render logic: If the return value from the getSnapshot value changes, then that is equivalent to a state change and then a re-render gets triggered.

NOTE

If the subscribe function changes between renders, then useSyncExternalStore will re-subscribe to the store. Similarly, if the value returned from getSnapshot change between renders, then React will trigger a re-render of the component because the state changed.

WARNING

If the value you return from getSnapshot is a reference value, like the object literal we're returning, and that reference value isn't memoized, then you'll throw React into an infinite loop scenario since useSyncExternalStore will re-render every time the reference value changes, and every re-render will call getSnapshot again.

There are two ways to mitigate this:

• return only primitive values from getSnapshot(): this way value- equality checking works
• cache any object values from getSnapshot(): this way value-equality works by returning the same object reference across renders.

Here's an example where by returning object values and instantiating the getSnapshot() and subscribe() functions inside the component, we have to use useMemo() and useCallback() to avoid constantly recreating those functions and thus creating infinite loops:

import * as React from "react"



export default function NetworkIndicator () {
	// 1. create "cache" of snapshot state so that reference stays consistent.
	const cache = React.useRef({
	    networkStatus: navigator.onLine ? "online" : "offline",
	    language: navigator.language
	})
	
	// 2. update "cache" to preserve object creation across re-renders
	const getSnapshot = React.useCallback(() => {
	  const prevState = cache.current
	
	  const nextState = {
	    networkStatus: navigator.onLine ? "online" : "offline",
	    language: navigator.language
	  }
	
	  if (
	    prevState.networkStatus === nextState.networkStatus &&
	    prevState.language === nextState.language
	  ) {
	    return prevState
	  }
	  
	  cache.current = nextState
	  return nextState
	}, [])

  // avoid recreating function
  const subscribe = React.useCallback((callback) => {
    console.log("Subscribing to network status")
    window.addEventListener("online", callback)
    window.addEventListener("offline", callback)

    return () => {
      console.log("Unsubscribing to network status")
      window.removeEventListener("online", callback)
      window.removeEventListener("offline", callback)
    }
  }, [])

  const networkStatus = React.useSyncExternalStore(
    subscribe, 
    getSnapshot
  )

  return (
    <div className="network">
      <span className={networkStatus} />
      <label>{networkStatus}</label>
    </div>
  )
}

Example

Here's an example where we naively use useState + useEffect for matching media:

import * as React from "react";
import { phone, desktop } from "./icons";

const query = "only screen and (max-width : 768px)";

export default function MatchMedia() {
  const [isMobile, setIsMobile] = React.useState(false);

  React.useEffect(() => {
    const handleChange = () => {
      setIsMobile(window.matchMedia(query).matches);
    };

    const matchMedia = window.matchMedia(query);

    matchMedia.addEventListener("change", handleChange);

    return () => {
      matchMedia.removeEventListener("change", handleChange);
    };
  }, []);

  return (
    <section>
      Resize your browser's window to see changes.
      <article>
        <figure className={isMobile ? "active" : ""}>
          {phone}
          <figcaption>Is mobile: {`${isMobile}`}</figcaption>
        </figure>

        <figure className={!isMobile ? "active" : ""}>
          {desktop}
          <figcaption>Is larger device: {`${!isMobile}`}</figcaption>
        </figure>
      </article>
    </section>
  );
}

Here is the imporved version using useSyncExternalStore:

import * as React from "react";
import { phone, desktop } from "./icons";

const query = "only screen and (max-width : 768px)";

const getSnapshot = () => {
  return window.matchMedia(query).matches
}

const subscribe = (callback) => {
  const matchMedia = window.matchMedia(query);

  matchMedia.addEventListener("change", callback);

  return () => {
    matchMedia.removeEventListener("change", callback);
  };
}

export default function MatchMedia() {
  const isMobile = React.useSyncExternalStore(subscribe, getSnapshot)


  return (
    <section>
      Resize your browser's window to see changes.
      <article>
        <figure className={isMobile ? "active" : ""}>
          {phone}
          <figcaption>Is mobile: {`${isMobile}`}</figcaption>
        </figure>

        <figure className={!isMobile ? "active" : ""}>
          {desktop}
          <figcaption>Is larger device: {`${!isMobile}`}</figcaption>
        </figure>
      </article>
    </section>
  );
}

useEffectEvent

useEffect recap

useEffect encapsulates side effect code by removing it from the rendering flow and then asynchronously executing that side effect code after rendering, when it's safe to do so.

Here is how the useEffect execution lifecycle works:

• after initial render: executes side effects after the first render of the component has completed, and queues up the cleanup function to execute before the first re-render.
• subsequent re-renders
  • without dependency array: executes the side effect code again after every render.
  • with dependency array: When one of the values in its dependency array has changed, useEffect will re-execute after the render, and then when one of its dependencies changes again, it queues up the cleanup function to execute before the next re-render.

useEffectEvent usage

Whenever you need to access a reactive but non-synchronizing value inside of useEffect, look into abstracting that logic into useEffectEvent.

// url is the only synchronizing value, we just use state.
const onPageView = React.useEffectEvent((url) => {
  pageview(url, state)
})

// synchronize only to url, not state
React.useEffect(() => {
  onPageView(url)
}, [url])

NOTE

The main use case for useEffectEvent is that is allows you to abstract reactive but non-synchronizing values out of useEffect, which eliminates unnecessary useEffect executions.

The main use case for using this hook is when you run into a problem like this:

React.useEffect(() => {
  const id = window.setInterval(() => {
    setCount((c) => c + step)
  }, delay)

  return () => window.clearInterval(id)
}, [delay, step])
// setting the interval doesn't really count on step,
// so we have unnecessary creation and teardown

We need to access the reactive step value from inside of useEffect, but we don't want to include it in the dependency array because it has nothing to do with setting and removing our interval.

This is the perfect use case for useEffectEvent. All we have to do is abstract our reactive but non-synchronizing logic into useEffectEvent, then we can use that event handler inside of useEffect without needing to include it as a dependency.

import * as React from "react"
import Slider from "./Slider"

React.useEffectEvent = React.experimental_useEffectEvent;

export default function App() {
  const [count, setCount] = React.useState(0)
  const [delay, setDelay] = React.useState(100)
  const [step, setStep] = React.useState(1)

  const handleDelayChange = (d) => setDelay(d)
  const handleStepChange = (s) => setStep(s)

  // step and count are reactive, but not used for synchronization for interval
  const onInterval = React.useEffectEvent(() => {
    setCount(count + step)
  })

  // in effect, only synchronizing state is delay
  React.useEffect(() => {
    const id = window.setInterval(onInterval, delay)

    return () => window.clearInterval(id)
  }, [delay])

  return (
    <main>
      <h1>{count}</h1>
      <Slider
        min={100}
        max={2000}
        step={100}
        onChange={handleDelayChange}
        label="ms delay"
      />
      <Slider
        min={1}
        max={10}
        step={1}
        onChange={handleStepChange}
        label="increment by"
      />
    </main>
  )
}

Concurrent React

When React is busy doing computationally expensive renders, it blocks the main thread and prevents other renders from happening, no matter how lightweight they might be. A big performance problem in React is that it treats all renders and computations with the same priority.

NOTE

The problem with old react

---

if React was rendering, and a high-priority event like a user input occurred, because all of React's work happened on the main thread, React would have no choice but to finish rendering before it could process that event.

Concurrent React is a way to schedule and prioritize different component renders as to prioritize high-priority renders and avoid blocking the main thread by offloading expensive yet low-priority rendering work to a separate background thread.

However, we as developers have to manually tell React which rendering work is high priority and which is low priority and can be deferred, which is where these two hooks come into play:

useDeferredValue

The useDeferredValue hook is a way to activate concurrent rendering in React by telling React to defer updating a value until all of its high priority work has finished.

NOTE

You can think of useDeferredValue as a smarter, more dynamic debouncing functionality for executing high-priority work, deferring updating state until all of the high priority rendering work has finished.

You can think of useDeferredValue as a way to tell React to defer updating a value until all its high-priority rendering work has finished.

For example, a common problem this hook was made to fix is a search items in a list functionality, where the query changes the list state, often performing an API call and then rendering all search results in the list. Here are the two rendering things in this problem:

• high priority, low computation: rendering the search term in the <input> tag
• low priority, expensive computation: rendering the list of results from search

Historically, React would treat updating the input field with the exact same priority as updating the expensive component. Often times, this meant the value, and therefore the text in the input field, would lag behind what the user had actually typed since React was too busy rendering the expensive list component to notice.

Instead, what we want is for React to always prioritize updating the input field, and then, only when it's finished, re-render the Employees component with the final value.

This is exactly how useDeferredValue works.

You pass it the value you want to defer, and React will automatically defer updating that value until all its high-priority rendering work has finished.

import * as React from "react"
import Employees from "./Employees"

export default function App() {
  // high priority user-facing search term
  const [searchQuery, setSearchQuery] = React.useState("");
  
  // low priority search term used to render list (expensive API call)
  const deferredSearchQuery = React.useDeferredValue(searchQuery)

  return (
    <div className="wrapper">
      <div className="search-container">
        <label>Filter Employees</label>
        <input
          type="text"
          placeholder="Laurie Lowe"
          className="search-input"
          value={searchQuery}
          onChange={(e) => setSearchQuery(e.target.value)}
        />
      </div>

      <Employees query={deferredSearchQuery} />
    </div>
  );
}

By passing the deferredSearchQuery to the Employees component, and wrapping Employees inside of React.memo, Employees will only re-render when deferredSearchQuery has changed, which is when React has already finished all of it's high-priority work.

useTransition

The useTransition hook also is another way to hook into concurrent rendering in React, by wrapping state updates that lead to computationally expensive operations (re-render may be expensive or state update may kick off expensive side effect) in a "transition".

Whenever you wrap a state update inside of React.startTransition, that tells React that that update, which we'll call a transition, may trigger a computationally expensive render, and as such, to not block any other higher-priority events that may occur during that render.

NOTE

A transition is a non-urgent, interruptible state update that allows React to defer updating the UI until all the work and the transition is complete.

NOTE

startTransition allows developers to mark specific state updates as lower priority. Any state updates wrapped in startTransition will be treated as non-blocking updates that can be interrupted if a higher-priority update occurs.

The way it works is while React is busy working on the transition, it will continue to show the user what they were already seeing, while at the same time, checking every 5ms to see if there are any other higher-priority events that it should prioritize.

If there are, it will pause the work it's doing and shift its priority.

If not, or once all the high-priority work has finished, it'll go back to working on the transition, committing the work all at once when it's finished – updating the UI.

This is how it works:

1. React queues state updates wrapped in a startTransition() call to take place, offloading its rendering work caused by that state update to another thread while showing the UI rendered by the previous snapshot (the snapshot before the transition executes)
2. While the transition is executing in the background, React checks every 5ms for high priority rendering work to run in the main thread
3. Once all high priority rendering work is finished, React finishes the transition and updates the UI with the state update snapshot the transition wrapped.

const [isPending, startTransition] = React.useTransition()

And here is a complete example:

import * as React from 'react';
import useConversations from './useConversations';
import Chat from './Chat';
import { MessageSquareIcon } from './icons';

export default function App() {
  const conversations = useConversations();
  const [activeChat, setActiveChat] = React.useState(0);

  const handleSelectChat = (index) => {
    React.startTransition(() => {
      setActiveChat(index);
    })
  };

  return (
    <div>
      <aside>
        <div>
          {conversations.map(({ id, messages, title }, i) => {
            let className = activeChat === i ? 'selected' : '';

            return (
              <button
                key={id}
                className={className}
                onClick={() => handleSelectChat(i)}
              >
                <MessageSquareIcon length={messages.length} />
                <span>{title}</span>
              </button>
            )
          })}
        </div>
      </aside>

      <Chat conversation={conversations[activeChat]} />
    </div>
  );
}

async startTransition

In concurrent react, we can also mark any type of computational work as low-priority, not just rendering work. For example, since going to the network is the main bottleneck in most web apps, we can mark that as low priority work to free up other high priority async work.

So in React v19, that's exactly what they did by giving transitions the ability to support async functions – and they even gave them a fun name, "actions".

We can use startTransition and pass in an async callback - also called action - to mark any code inside that callback as low-transition code.

const action = async () => {}

startTransition(action)

NOTE

The main benefit of using async transitions and actions over managing loading state manually is that all currently queued async transition calls are batched together and only update the UI when every transition has finished. Think of it as a really good debounce.

However, there's one major caveat: any state update code inside an async startTransition callback will NOT be treated as a low-priority state update and thus will not be considered a "transition". To fix this, just wrap that state update in a nested startTransition() call.

WARNING

When you use await inside a startTransition function, the state updates that happen after the await are not marked as Transitions.

import * as React from "react"
import { fetchPrice } from "./api"

export default function App() {
  const [quantity, setQuantity] = React.useState(1);
  const [price, setPrice] = React.useState(null)
  const [isPending, startTransition] = React.useTransition()

  React.useEffect(() => {
    const handleFetchPrice = async () => {
      const data = await fetchPrice(quantity)
      setPrice(data)
    }

    handleFetchPrice()
  }, [])

  const handleChange = async (e) => {
    const nextQty = Number(e.target.value)
    setQuantity(nextQty);
    
    startTransition(async () => {
      const data = await fetchPrice(nextQty)
      startTransition(() => {
        setPrice(data)
      })
    })
  }

  if (!price) {
    return null
  }

  const baseUrl = `https://ui.dev/api/checkout?product_id=subscription`

  return (
    <main>
      <img
        src="https://ui.dev/images/subscribe/tier-annual.svg"
        alt="ui.dev courses banner"
      />
      <div>
        <em>
          {price.discountedPrice || price.basePrice} <span>/yr</span>
        </em>
        {price.discountedPrice && <s>{price.basePrice}</s>}
      </div>

      <div>
        Team Size
        <input type="number" value={quantity} onChange={handleChange} />
      </div>

      <p>
        {price.discount?.percentOff > 0 
          ? <>Enjoy your <strong>{price.discount.percentOff}% off</strong> bulk discount <br /></> 
          : null}
        {price.message}
      </p>

      <a
        href={`${baseUrl}&quantity=${quantity}&code=${price.discount?.code || ""}`}
        className={isPending ? "pending" : ""}
      >
        Subscribe to ui.dev
      </a>
    </main>
  );
}

useOptimistic

The useOptimistic hook is built on top of React's transition architecture, and it gives you access to the intermediate state updates during a transition, whereas useTransition does not. It only gives you the final state after batching all transitions

here is the basic structure of using it:

const [optimisticState, addOptimistic] = useOptimistic(
  state,
  (currentState, optimisticValue) => {
    return getNextOptimisticState(currentState, optimisticValue);
  }
)

Let's break it down:

• optimisticState: set to the initial state initially, but will be whatever is returned from the callback you passed in, when you execute that callback using addOptimistic.
• addOptimistic(): executes the callback you passed in.

Here is an example of using this hook:

import * as React from "react";
import useNotes from "./useNotes";

export default function FieldNotes() {
  const { notes, addNote } = useNotes();

  const [optimisticNotes, addOptimisticNote] = React.useOptimistic(
    notes,
    (state, newNote) => [
      ...state,
      {
        text: newNote,
        status: "pending"
      },
    ]
  );

  const formAction = async (formData) => {
    const newNote = formData.get("note");

    if (newNote.trim()) {
      addOptimisticNote(newNote);
      await addNote(newNote);
    }
  }

  return (
    <article>
      <h1>Field Notes</h1>
      <div>
        <ul>
          {optimisticNotes.map((note, index) => (
            <li key={index}>
              <p>{note.text}</p>
              {note.status === "pending" && <i>saving...</i>}
            </li>
          ))}
        </ul>
        <form action={formAction}>
          <input
            required
            type="text"
            name="note"
            placeholder="Type your note..."
          />
          <button className="link" type="submit">
            Submit
          </button>
        </form>
      </div>
    </article>
  );
}

Here's a more advanced way of using this hook, where we account for all CRUD ways to optimistically update a list of objects.

function TodoList() {
  const [todos, setTodos] = useState([]);

  const [optimisticTodos, setOptimisticTodos] = React.useOptimistic(
    todos,
    (currentTodos, action) => {
      switch (action.type) {
        case "add":
          return [...currentTodos, action.todo];
        case "delete":
          return currentTodos.filter((todo) => todo !== action.todo);
        case "update":
          return currentTodos.map((todo) =>
            todo === action.oldTodo ? action.newTodo : todo
          );
        default:
          return currentTodos;
      }
    }
  );
}

use

The use() function in React is not a hook but rather it is a way to await promises but with two important behaviors:

1. transitions with use(promise) will be considered pending: if you call use(promise) in a transition of any kind, that transition will be considered pending until the promise resolves.
2. React rerenders when use(promise) resolves: If you resolve a promise with use(), React re-renders even though it's not a state update.

To avoid infinite re-rendering when using use(promise) (since a promise is an object, each re-render leads to a different in-memory address for that promise), it's important to cache promises somehow:

WARNING

use lets you unwrap the value of a promise, but you need to make sure that promise is referentially consistent across renders. This is usually done by caching the promise yourself.

import * as React from "react"
import { fetchPokemon } from "./api"

let cache = new Map()

// use cache for stable references instead of constantly recreating promises
const getPokemon = (id) => {
  if (!cache.has(id)) {
    cache.set(id, fetchPokemon(id))
  }

  return cache.get(id)
}

export default function PokemonCard({ id }) {
  // blocks component from rendering until promise resolves
  const pokemon = React.use(getPokemon(id))

  return (
    <div className="card">
      <figure>
        <img
          width="475px"
          height="475px"
          src={pokemon?.sprites?.front_default}
          alt={pokemon.name}
        />
        <figcaption>
          <h4>{pokemon.name}</h4>
          <h6>No. {pokemon.id}</h6>
        </figcaption>
      </figure>
    </div>
  )
}

Whenever we use use(promise) in a component, the component itself is considered "pending" and thus we can wrap it in a <Suspense> to show some loading state until the promise resolves and thus the component is ready to be rendered.

import * as React from "react"
import PokemonCard from "./PokemonCard"
import Carousel from "./Carousel"
import LoadingCard from "./LoadingCard"

export default function App () {
  const [id, setId] = React.useState(1)
  const [isPending, startTransition] = React.useTransition()

  const handlePrevious = () => {
    if (id > 1) {
      startTransition(() => {
        setId((currentId) => currentId - 1)
      })
    }
  }

  const handleNext = () => startTransition(() => {
    setId((currentId) => currentId + 1)
  })

  return (
    <Carousel onPrevious={handlePrevious} onNext={handleNext}>
      <React.Suspense fallback={<LoadingCard />}>
        {isPending ? <LoadingCard /> : <PokemonCard id={id} />}
      </React.Suspense>
    </Carousel>
  )
}

Form actions

As of react 19, instead of passing an onSubmit= handler to a <form> element, we can pass an action instead, which has several benefits:

1. cleaner code
2. no need for controlled form inputs: we automatically get the FormData instance that contains the values of all the fields in the form at the time of submission.

export default function Search() {
  const searchAction = async (formData) => {
    const query = formData.get("query");
    const results = await submitSearch(query);
    redirectTo("/results", { results });
  }

  return (
    <form action={searchAction}>
      <input name="query" />
      <button type="submit">Search</button>
    </form>
  );
}

Server components

Server components enable cool shit like this:

NOTE

Components with the use client directive can (and do with certain frameworks) render on a server. A better way to think about it is Server Components generate serializable templates and use client generates a JavaScript bundle.

whenever you add the use client directive to a module, React will treat that module and all of its transitive dependencies as Client modules (and include them in the bundle it generates).

101 Tips

1. HOC

A higher order component transforms a component into a better version of itself, adding additional functionality to it. It is a function that takes in a component and returns a new component

HOC are named with the with convention, as prefixed function started with “with” as a way of saying that HOCs enhance components with some funcitonality.

Here are some rules for creating a good higher order component

1. Don’t mutate the passed-in component
2. Don’t destroy the structure of props for the passed-in component
   • Spread across all the props, and then you can add new ones.
3. Don’t instantiate HOCs inside other components
   • Instantiating an enhanced component inside another component makes it vulnerable to re-renders.
   • Only instantiate HOCs outside components.

   const Component = (props) => {
     // This is wrong. Never do this
     const EnhancedComponent = HOC(WrappedComponent);
     return <EnhancedComponent />;
   };
   
   // This is the correct way
   const EnhancedComponent = HOC(WrappedComponent);
   const Component = (props) => {
     return <EnhancedComponent />;
   };

Here is an example of an HOC:

function withMousePosition(WrappedComponent) {
	return (props) => {
		const [mouseX, setMouseX] = useState(0); 
		const [mouseY, setMouseY] = useState(0); 

		useEffect(() => {
			const handleMouseMove = (e) => {
				setMouseX(e.clientX)
				setMouseY(e.clientY)
			}

			window.addEventListener("mousemove", handleMouseMove)
			return () => window.removeEventListener("mousemove", handleMouseMove)
		}, [])

		return <WrappedComponent {...props} mouseX={mouseX}, mouseY={mouseY} />
	}
}

2. Dealing with Form state

The best way to deal with form state is to use object state and then create handler that sets state using event.target.value.

function Form() {
  const [{ name, email }, setFormState] = useState({
    name: "",
    email: "",
  });

  const createFormValueChangeHandler = (field) => {
    return (event) => {
      setFormState((formState) => ({
        ...formState,
        [field]: event.target.value,
      }));
    };
  };

  return (
    <>
      <h1>Class Registration Form</h1>
      <form>
        <label>
          Name:{" "}
          <input
            type="text"
            value={name}
            onChange={createFormValueChangeHandler("name")}
          />
        </label>
        <label>
          Email:{" "}
          <input
            type="email"
            value={email}
            onChange={createFormValueChangeHandler("email")}
          />
        </label>
      </form>
    </>
  );
}

And here is the custom hook version of that:

import React, { useState } from "react";

export function useFormObjectState<T extends Record<string, any>>(
  initialData: T
) {
  const [formState, setFormState] = useState(initialData);

  const createFormValueChangeHandler = (field: keyof T) => {
    return (event: React.ChangeEvent<HTMLInputElement>) => {
      setFormState((formState) => ({
        ...formState,
        [field]: event.target.value,
      }));
    };
  };

  return { formState, createFormValueChangeHandler };
}

3. Master useRef

Did you know you can pass a function to the ref attribute instead of a ref object?

Here's how it works:

• When the DOM node is added to the screen, React calls the function with the DOM node as the argument.
• When the DOM node is removed, React calls the function with null.

Here is how we can use refs with a function call, where we pass in a callback to the ref= prop on a JSX element, where this callback has one argument, which is the DOM element we're attaching the ref to.

This pattern is useful to prevent us from doing a useEffect to access and do stuff on the ref - instead the function is called whenever the ref is available or null.

❌ Before: Using useEffect to focus the input

function App() {
  const ref = useRef();

  useEffect(() => {
    ref.current?.focus();
  }, []);

  return <input ref={ref} type="text" />;
}

✅ After: We focus on the input as soon as it is available.

function App() {
  // this function will get executed as soon as the <input> gets mounted
  const ref = useCallback((inputNode) => {
    inputNode?.focus();
  }, []);

  return <input ref={ref} type="text" />;
}

useRef for persistent values without triggering re-renders

You can also use useRef as a way to keep the value of a variable persisting across re-renders.

function Timer() {
  const [time, setTime] = useState(new Date());
  const intervalIdRef = useRef();
  const intervalId = intervalIdRef.current;

  useEffect(() => {
    const interval = setInterval(() => {
      setTime(new Date());
    }, 1_000);
    intervalIdRef.current = interval;
    return () => clearInterval(interval);
  }, []);

  const stopTimer = () => {
    intervalId && clearInterval(intervalId);
  };

  return (
    <>
      <>Current time: {time.toLocaleTimeString()} </>
      <button onClick={stopTimer}>Stop timer</button>
    </>
  );
}

forward refs

You can also use forward refs to allow passing a ref to a custom component you made.

import React from 'react'

const LabelledInput = (props, ref) => {
  const { id, label, value, onChange } = props

  return (
    <div class="labelled--input">
      <label for={id}>{label}</label>
      <input id={id} onChange={onChange} value={value} ref={ref}/>
    </div>
  )
}

export default React.forwardRef(LabelledInput)

The last thing you need to understand is that to prevent memory leaks, you need to clean up refs in the useEffect cleanup.

import React, { useRef, useEffect } from 'react';

function MyComponent() {
  const myRef = useRef();

  useEffect(() => {
    // Cleanup when the component unmounts
    return () => {
      myRef.current = null;
    };
  }, []);

  return (
    <div>
      <h1>My Component</h1>
      <div ref={myRef}>This is a DOM element.</div>
    </div>
  );
}

export default MyComponent;

4. Don't create state unnecessarily

If you can derive a value from state already set up in your component, then just do that. Don't create another state - just use useMemo.

11. When storing the selected item from a list, store the item ID rather than the entire item

❌ Bad: If an item is selected but then it changes (i.e., we receive a completely new object reference for the same ID), or if the item is no longer present in the list, selectedItem will either retain an outdated value or become incorrect.

function ListWrapper({ items }) {
  // We are referencing the entire item
  const [selectedItem, setSelectedItem] = useState<Item | undefined>();

  return (
    <>
      {selectedItem != null && <div>{selectedItem.name}</div>}
      <List
        items={items}
        selectedItem={selectedItem}
        onSelectItem={setSelectedItem}
      />
    </>
  );
}

✅ Good: We store the selected item by its ID (which should be stable). This ensures that even if the item is removed from the list or one of its properties changed, the UI should be correct.

function ListWrapper({ items }) {
  const [selectedItemId, setSelectedItemId] = useState<number | undefined>();
  // We derive the selected item from the list
  const selectedItem = items.find((item) => item.id === selectedItemId);

  return (
    <>
      {selectedItem != null && <div>{selectedItem.name}</div>}
      <List
        items={items}
        selectedItemId={selectedItemId}
        onSelectItem={setSelectedItemId}
      />
    </>
  );
}

React + Typescript

ComponentProps

The ComponentProps<> generic type allows you to get the props of a component easily. You can do this with HTML elements or other components:

• ComponentProps<"button">: returns the prop types of a <button> element
• ComponentProps<typeof MyComponent>: returns the prop types of a custom component you have.

const ButtonWithLogging = (props: ComponentProps<"button">) => {
  const handleClick: MouseEventHandler<HTMLButtonElement> = (e) => {
    console.log("Button clicked"); //TODO: Better logging
    props.onClick?.(e);
  };
  return <button {...props} onClick={handleClick} />;
};

Events

Here are the types for events and event handlers:

event object e typing

• MouseEvent<T>: for "click" event
• FocusEvent<T>: for "focus" event
• ChangeEvent<T>: for "change" event

const MyComponent = ({ onClick, onFocus, onChange }: {
  onClick: (e: MouseEvent<HTMLButtonElement>) => void;
  onFocus: (e: FocusEvent<HTMLButtonElement>) => void;
  onChange: (e: ChangeEvent<HTMLInputElement>) => void;
}) => {
  //   …
};

event handler

• MouseEventHandler<T>: event handler for click event
• FocusEventHandler<T>: event handler for "focus" event
• ChangeEventHandler<T>: event handler for change event

const MyComponent = ({ onClick, onFocus, onChange }: {
  onClick: MouseEventHandler<HTMLButtonElement>;
  onFocus: FocusEventHandler<HTMLButtonElement>;
  onChange: ChangeEventHandler<HTMLInputElement>;
}) => {
  //   …
};

React best practices

title: "GitHub - davidkpiano/frontend-masters-state-workshop"
image: "https://opengraph.githubassets.com/d31d699dbb499b07d4056766d19c8d3f58be25dfca69ae3be31d8d82e83ea43c/davidkpiano/frontend-masters-state-workshop"
description: "Contribute to davidkpiano/frontend-masters-state-workshop development by creating an account on GitHub."
url: "https://github.com/davidkpiano/frontend-masters-state-workshop"
favicon: ""
aspectRatio: "50"

Antipatterns

Here are the three antipatterns you should look out for and how to fix them:

1. Fix Derived State Anti-patterns

   • Remove unnecessary state and effects
   • Calculate derived values directly in render
   • Use useMemo only when needed, when computation is expensive.

2. Convert useState to useRef

   • Identify values that don't need re-renders
   • Replace useState with useRef where appropriate
   • Ensure cleanup is handled properly

3. Eliminate Redundant State

   • Store minimal required state
   • Derive values instead of duplicating state
   • Use proper data normalization

Deriving state antipattern

• Rule: If you can calculate (derive) it, don't store it
• Anti-pattern: Using useState + useEffect to sync derived data
• Best practice: Calculate derived values directly in render or with useMemo
• Benefits:
  • Eliminates synchronization bugs
  • Reduces state complexity
  • Automatically stays in sync with source data
• Examples of derived state:
  • Filtered/sorted lists from original data + criteria
  • Computed totals from item arrays
  • Status calculations from multiple boolean flags
  • Available items from excluded items + full list
• When to useMemo: Only when the calculation is expensive and dependencies change infrequently

Before (Anti-pattern):

function TripSummary() {
  const [tripItems] = useState([
    { name: 'Flight', cost: 500 },
    { name: 'Hotel', cost: 300 },
  ]);
  const [totalCost, setTotalCost] = useState(0); // ❌ Unnecessary state

  useEffect(() => {
    setTotalCost(tripItems.reduce((sum, item) => sum + item.cost, 0)); // ❌ Sync effect
  }, [tripItems]);

  return <div>Total: ${totalCost}</div>;
}

After (Best practice):

function TripSummary() {
  const [tripItems] = useState([
    { name: 'Flight', cost: 500 },
    { name: 'Hotel', cost: 300 },
  ]);

  // ✅ Derive the value directly
  const totalCost = tripItems.reduce((sum, item) => sum + item.cost, 0);

  return <div>Total: ${totalCost}</div>;
}

Refs vs state

• Rule: Use useRef for values that don't affect rendering
• Anti-pattern: Using useState for mutable values that don't need re-renders
• Best practice: useRef for DOM references, timers, counters, previous values
• Key differences:
  • useState: Triggers re-render when changed
  • useRef: No re-render when .current changes
• Common use cases:
  • Timer IDs (setInterval/setTimeout)
  • Scroll position tracking
  • Analytics/tracking data
  • Caching expensive calculations
  • Storing previous prop values

Before (Anti-pattern):

function Timer() {
  const [timeLeft, setTimeLeft] = useState(60);
  const [timerId, setTimerId] = useState<NodeJS.Timeout | null>(null); // ❌ Causes re-renders

  const startTimer = () => {
    const id = setInterval(() => {
      setTimeLeft((prev) => prev - 1);
    }, 1000);
    setTimerId(id); // ❌ Triggers unnecessary re-render
  };

  useEffect(() => {
    return () => timerId && clearInterval(timerId);
  }, [timerId]); // ❌ Effect runs every time timerId changes

  return <div>{timeLeft}s remaining</div>;
}

After (Best practice):

function Timer() {
  const [timeLeft, setTimeLeft] = useState(60);
  const timerIdRef = useRef<NodeJS.Timeout | null>(null); // ✅ No re-renders

  const startTimer = () => {
    const id = setInterval(() => {
      setTimeLeft((prev) => prev - 1);
    }, 1000);
    timerIdRef.current = id; // ✅ No re-render triggered
  };

  useEffect(() => {
    return () => timerIdRef.current && clearInterval(timerIdRef.current);
  }, []); // ✅ Effect runs only once

  return <div>{timeLeft}s remaining</div>;
}

Redundant State

Basically, do not store arrays of entire javascript objects within state, as this has increased memory usage. Rather, just store the IDs, and then lookup the full object when needed.

• Rule: Single source of truth for each piece of data
• Anti-pattern: Storing the same data in multiple places
• Best practice: Store minimal state, derive everything else
• Common redundancy patterns:
  • Storing full objects when only ID is needed
  • Duplicating data already available in props/context
  • Storing both raw and formatted versions of same data
  • Keeping derived calculations in separate state
• Problems with redundant state:
  • Synchronization bugs when data gets out of sync
  • More complex update logic
  • Increased memory usage
  • Harder to debug and maintain
• Solutions:
  • Store only IDs, look up full objects when needed
  • Use props/context data directly
  • Format data during render, not in state

Before (Anti-pattern):

function HotelSelection() {
  const [hotels] = useState([
    { id: 'h1', name: 'Grand Hotel', price: 200 },
    { id: 'h2', name: 'Budget Inn', price: 80 },
  ]);
  const [selectedHotel, setSelectedHotel] = useState<Hotel | null>(null); // ❌ Stores entire object

  const handleSelect = (hotel: Hotel) => {
    setSelectedHotel(hotel); // ❌ Duplicates data from hotels array
  };

  return (
    <div>
      {selectedHotel && (
        <div>
          {selectedHotel.name} - ${selectedHotel.price}
        </div>
      )}
    </div>
  );
}

After (Best practice):

function HotelSelection() {
  const [hotels] = useState([
    { id: 'h1', name: 'Grand Hotel', price: 200 },
    { id: 'h2', name: 'Budget Inn', price: 80 },
  ]);
  const [selectedHotelId, setSelectedHotelId] = useState<string | null>(null); // ✅ Store only ID

  const handleSelect = (hotelId: string) => {
    setSelectedHotelId(hotelId); // ✅ Store minimal data
  };

  // ✅ Derive the full object when needed
  const selectedHotel = hotels.find((h) => h.id === selectedHotelId);

  return (
    <div>
      {selectedHotel && (
        <div>
          {selectedHotel.name} - ${selectedHotel.price}
        </div>
      )}
    </div>
  );
}

Cascading use effects

Cascading use effects where each effect not only triggers a re-render but another effect leads to buggy, hard-to-trace code and is a complete antipattern

// Effect 1: Trigger search when inputs change
useEffect(() => {
  if (destination && startDate && endDate) {
    setIsSearchingFlights(true);
  }
}, [destination, startDate, endDate]);

// Effect 2: Perform flight search
useEffect(() => {
  if (!isSearchingFlights) return;
  // ... search logic
}, [isSearchingFlights]);

// Effect 3: Trigger hotel search when flight selected
useEffect(() => {
  if (selectedFlight) {
    setIsSearchingHotels(true);
  }
}, [selectedFlight]);

// Effect 4: Perform hotel search
useEffect(() => {
  if (!isSearchingHotels) return;
  // ... search logic
}, [isSearchingHotels]);

Here is a better solution using a reducer, which allows for only one useEffect where we run different code based on the state from the reducer:

type Action =
  | { type: 'SET_INPUT'; inputs: Partial<SearchInputs> }
  | { type: 'flightUpdated'; flight: Flight }
  | { type: 'hotelUpdated'; hotel: Hotel }
  | { type: 'SET_ERROR'; error: string };

function reducer(state: BookingState, action: Action): BookingState {
  switch (action.type) {
    case 'SET_INPUT':
      const inputs = { ...state.inputs, ...action.inputs };
      return {
        ...state,
        inputs,
        status: allInputsValid(inputs) ? 'searchingFlights' : state.status,
      };
    case 'flightUpdated':
      return {
        ...state,
        status: 'searchingHotels',
        selectedFlight: action.flight,
      };
    // ... other cases
  }
}

// Single effect handles all async operations based on status
useEffect(() => {
  if (state.status === 'searchingFlights') {
    searchFlights().then((flight) =>
      dispatch({ type: 'flightUpdated', flight })
    );
  }
  if (state.status === 'searchingHotels') {
    searchHotels().then((hotel) => dispatch({ type: 'hotelUpdated', hotel }));
  }
}, [state]);

Deeply nested data

• Rule: Flatten data structures by storing entities in separate collections with ID references
• Anti-pattern: Deep nesting creates complex dependencies and update patterns
• Best practice: Normalize data to avoid redundancy and ensure consistency
• Benefits:
  • Simplified updates with O(1) lookups instead of O(n+m) traversals
  • Better performance with minimal re-renders
  • Cleaner, more maintainable reducer logic
  • Easier implementation of cross-entity operations

Deeply nested data structures like so where an object has a sub-collection (an array of objects) lead to O(n x m) updates and traversals, leading to less performant code.

// ❌ Nested structure
interface NestedState {
  destinations: Array<{
    id: string;
    name: string;
    todos: Array<{
      id: string;
      text: string;
    }>;
  }>;
}

This is bad. for example, to find a specific todo within a specific destination, the code complexity would be O(n + m):

function findTodo(state: NestedState, destinationId: string, todoId: string) {
	// O(n)
	const destination = state.destinations.find(
		dest => dest.id === destinationId
	)
	// O(m)
	const todo = destination.todos.find(todo => todo.id === todoId)
	return todo // O (n + m)
}

You can normalize it better, like so, modeling relational tables like SQL, and instead of using arrays, you can model a collection as a large, flat object, where each key in a collection object is an ID that maps to the actual record, resulting in O(1) lookup, especially when used with a map.

// ✅ Normalized structure
interface NormalizedState {
  destinations: { [id: string]: { id: string; name: string } };
  todos: { [id: string]: { id: string; text: string; destinationId: string } };
}

Now to find a specific todo, the code becomes O(1):

function findDestinationOfTodo(state: NestedState, todoId: string) {
	const destinationId = state.todo[todoId].destinationId // O(1)
	return state.destinations[destinationId] // O(1)
}

Deeply Nested Updates

The current travel itinerary application stores data in a deeply nested structure where each destination contains an array of todos. This creates several problems:

When updating or deleting a todo item, the reducer must:

1. Find the correct destination by mapping through all destinations
2. Find the correct todo within that destination's todos array
3. Create a new nested structure preserving immutability

// ❌ Complex nested update - hard to read and error-prone
destinations: state.destinations.map((dest) =>
  dest.id === action.destinationId
    ? {
        ...dest,
        todos: dest.todos.filter((todo) => todo.id !== action.todoId),
      }
    : dest
);

Performance Issues

• O(n×m) complexity: Every todo operation requires iterating through destinations AND todos
• Unnecessary re-renders: Updating one todo causes the entire destinations array to be recreated
• Memory overhead: Deeply nested objects are harder for JavaScript engines to optimize

Code Complexity

• Reducer logic becomes increasingly complex with more nesting levels
• Difficult to implement features like global todo search or cross-destination operations
• Error-prone when adding new nested relationships

benefits of data normalization

Normalization flattens the data structure by storing entities in separate collections and using IDs to reference relationships:

Simplified Updates

// ✅ Normalized - direct and clear
case 'DELETE_TODO':
  return {
    ...state,
    todos: state.todos.filter(todo => todo.id !== action.todoId)
  }

Better Performance

• O(1) lookups: Direct access to entities by ID using objects/Maps
• Minimal re-renders: Only affected components re-render
• Efficient operations: No need to traverse nested structures

Code Clarity

• Each entity type has clear, focused update logic
• Easy to implement complex queries and cross-entity operations
• Reducer actions become more predictable and testable

---

When to use reducers

When dealing with a lot of event driven code, then reach for reducers. If we have a bunch of useEffects, each triggering the other, that's an antipattern and we should reach for a reducer.

1. Effects should be minimal: Use effects only for synchronizing with external systems
2. State updates should be explicit: Use actions that clearly describe what happened
3. Business logic belongs in reducers: Keep effects simple and focused
4. Think in events, not reactions: Model user interactions and business events

A good use case for reducers is to expose it in a context provider so that state is shared across your app with easy ways to update the context state.

// Context eliminates prop drilling
const BookingContext = createContext();

function BookingProvider({ children }) {
  const [state, dispatch] = useReducer(bookingReducer, initialState);
  return (
    <BookingContext.Provider value={{ state, dispatch }}>
      {children}
    </BookingContext.Provider>
  );
}

function FlightForm() {
  const { state, dispatch } = useBooking(); // Direct access to state
}

You can then use the context like so:

// Custom hook with validation
function useBooking() {
  const context = use(BookingContext);
  if (!context) {
    throw new Error('useBooking must be used within BookingProvider');
  }
  return context;
}

Another use case for reducers is to avoid cascading use effects and isntead just abstract that into one useEffect that relies on the state of the reducer.

type Action =
  | { type: 'SET_INPUT'; inputs: Partial<SearchInputs> }
  | { type: 'flightUpdated'; flight: Flight }
  | { type: 'hotelUpdated'; hotel: Hotel }
  | { type: 'SET_ERROR'; error: string };

function reducer(state: BookingState, action: Action): BookingState {
  switch (action.type) {
    case 'SET_INPUT':
      const inputs = { ...state.inputs, ...action.inputs };
      return {
        ...state,
        inputs,
        status: allInputsValid(inputs) ? 'searchingFlights' : state.status,
      };
    case 'flightUpdated':
      return {
        ...state,
        status: 'searchingHotels',
        selectedFlight: action.flight,
      };
    // ... other cases
  }
}

// Single effect handles all async operations based on status
useEffect(() => {
  if (state.status === 'searchingFlights') {
    searchFlights().then((flight) =>
      dispatch({ type: 'flightUpdated', flight })
    );
  }
  if (state.status === 'searchingHotels') {
    searchHotels().then((hotel) => dispatch({ type: 'hotelUpdated', hotel }));
  }
}, [state]);

A main use case of reducers is avoiding

Pure functions for app logic

• Rule: All business logic should be represented in pure functions
• Anti-pattern: Mixing side effects with state updates
• Best practice: Separate pure state transitions from side effects
• Benefits:
  • Deterministic behavior - same input always produces same output
  • Easy to test in isolation
  • Composable and reusable logic
  • Better performance through memoization

State machines for modeling (using status)

• Rule: Make impossible states impossible through explicit state modeling
• Anti-pattern: Using multiple boolean flags that can create invalid combinations
• Best practice: Use state machines to define valid states and transitions
• Benefits:
  • Prevents impossible states at compile time
  • Makes state transitions clear and predictable
  • Better error handling and edge case management
  • Self-documenting state logic

When trying to conditionally render things based on conditions with many boolean states, you can often arrive at a bug or an impossible state. By instead changing the boolean condition state to string enums, we explicitly say what types of state are possible and what we want to handle.

• Rule: Use discriminated unions to model different application states
• Anti-pattern: Using boolean flags that can create impossible states
• Best practice: Define explicit states with their associated data
• Benefits:
  • Impossible states become impossible
  • Type safety ensures correct data access
  • Clearer component logic
  • Better error handling

// ❌ Boolean flags can create impossible states
const [isLoading, setIsLoading] = useState(false);
const [error, setError] = useState(null);
const [data, setData] = useState(null);

// ✅ Type states prevent impossible combinations
type State =
  | { status: 'idle' }
  | { status: 'loading' }
  | { status: 'error'; error: string }
  | { status: 'success'; data: FlightData };

const [state, setState] = useState<State>({ status: 'idle' });

You can also take this one step further and take advantage of discriminated unions in typescript to ensure correct state updates. For example, you can update state based on the status and create a type like this:

• Notice the object overloads, where a unique object is created based on the status property.

type FlightData = {
  destination: string;
  departure: string;
  arrival: string;
  passengers: number;
  error: string | null;
} & (
  | {
      status: 'idle';
      flightOptions: null;
    }
  | {
      status: 'submitting';
      flightOptions: null;
    }
  | {
      status: 'error';
      error: string;
    }
  | {
      status: 'success';
      flightOptions: FlightOption[];
    }
);

Combining Related State

• Rule: Group related state variables into single objects for better organization
• Anti-pattern: Having many individual useState calls for related data
• Best practice: Combine related state into objects and use single state updates
• Benefits:
  • Fewer state variables to manage
  • Atomic updates ensure consistency
  • Easier to understand relationships between data
  • Less boilerplate code for state management

Before:

// ❌ Multiple individual states for related data
const [destination, setDestination] = useState('');
const [departure, setDeparture] = useState('');
const [arrival, setArrival] = useState('');
const [passengers, setPassengers] = useState(1);

// Updating a single field
setDestination('Paris');

After:

// ✅ Combined related state
const [searchForm, setSearchForm] = useState({
  destination: '',
  departure: '',
  arrival: '',
  passengers: 1,
});

// Updating a single field
setSearchForm({
  ...searchForm,
  destination: 'Paris',
});

setSearchForm((prev) => ({
  ...prev,
  destination: 'Paris',
}));

External state management in react

When to use external state management libraries in react? You use it when you have more than one react context provider in your app, which signals that your app is growing in complexity.

Context is not that performant, since it was meant for states that don't frequently change.

NOTE

In summary, choose libraries like zustand or nanostores over context when you have lots of state (multiple context providers) and state that changes frequently

There are two types of state management systems: stores and atoms.

stores vs atom

Store-based solutions (Zustand, Redux Toolkit, XState Store) use a centralized approach - all state lives in one or few stores.

Atomic solutions (Jotai, Recoil, XState Store) use a distributed approach - state is broken into independent atoms that can be composed.

Choose stores when you have:

• Complex state relationships - Many pieces of state depend on each other
• Clear data flow requirements - You need predictable, traceable state updates
• Team coordination needs - Multiple developers working on shared state logic

Store benefits:

• Single source of truth
• Excellent debugging with dev tools
• Clear separation of business logic
• Predictable state updates
• Great TypeScript support

Choose atoms when you have:

• External state - State is updated from an external source
• Independent pieces of state - Most state doesn't depend on other state
• Component-specific concerns - State is primarily tied to specific UI components
• Performance-critical applications - Need fine-grained subscriptions

Atomic benefits:

• Automatic optimization and caching
• Excellent performance with selective rendering
• Highly composable and reusable
• Bottom-up architecture flexibility
• Natural code splitting

You can also combine both approaches:

• Use stores for core business logic and complex workflows
• Use atoms for UI-specific state and independent pieces of data

// Core business logic in store
const useBookingStore = create<BookingStore>(...);

// UI-specific state as atoms
const themeAtom = atom<'light' | 'dark'>('light');
const sidebarOpenAtom = atom<boolean>(false);

NOTE

In summary, use atoms for simple state (user settings) and use stores for the main state of your application (user data, things with CRUD operations)

Xstate

Here is an example of creating a store in xstate:

import { createStore } from '@xstate/store';
import { useSelector } from '@xstate/store/react';

const bookingStore = createStore({
  context: initialState,
  on: {
    flightSearchUpdated: (
      context,
      event: Partial<{
        destination: string;
        departure: string;
        arrival: string;
        passengers: number;
        isOneWay: boolean;
      }>
    ) => {
      return {
        ...context,
        flightSearch: { ...context.flightSearch, ...event },
      };
    },
    searchFlights: (context) => {
      return {
        ...context,
        currentStep: Step.FlightResults,
      };
    },
    flightSelected: (context, event: { flight: FlightOption }) => {
      return {
        ...context,
        selectedFlight: event.flight,
        currentStep: Step.HotelSearch,
      };
    },
    changeFlight: (context) => {
      return {
        ...context,
        currentStep: Step.FlightSearch,
      };
    },
    hotelSearchUpdated: (
      context,
      event: Partial<{
        checkIn: string;
        checkOut: string;
        guests: number;
        roomType: string;
      }>
    ) => {
      return {
        ...context,
        hotelSearch: { ...context.hotelSearch, ...event },
      };
    },
    searchHotels: (context) => {
      return {
        ...context,
        currentStep: Step.HotelResults,
      };
    },
    hotelSelected: (context, event: { hotel: HotelOption }) => {
      return {
        ...context,
        selectedHotel: event.hotel,
        currentStep: Step.Review,
      };
    },
    changeHotel: (context) => {
      return {
        ...context,
        currentStep: Step.HotelSearch,
      };
    },
    book: (context) => {
      return {
        ...context,
        currentStep: Step.Confirmation,
      };
    },
    back: (context) => {
      switch (context.currentStep) {
        case Step.FlightResults:
          return {
            ...context,
            currentStep: Step.FlightSearch,
          };
        case Step.HotelSearch:
          return {
            ...context,
            currentStep: Step.FlightResults,
          };
        case Step.HotelResults:
          return {
            ...context,
            currentStep: Step.HotelSearch,
          };
        case Step.Review:
          return {
            ...context,
            currentStep: Step.HotelResults,
          };
        default:
          return context;
      }
    },
  },
});

Performance

Lazy Loading/Code splitting

Basic Code Splitting

Code splitting refers to the practice of requesting JavaScript only when you need it or the user interacts with it. It prevents a large JS bundle from slowing down initial page load.

Here is how you can asynchronously load React components and code split them:

1. Import a component asynchronously with the React.lazy() method

   import {lazy} from "React"
   
   const DetailsPage = lazy(() => import("./Details"))

2. Wrap the lazy-loaded component in a <Suspense> component, and provide the loading element to render in the fallback= prop.

   const Dashboard = () => {
   	return (
   		<Suspense>
   			<DetailsPage />
   		</Suspense>
   	)
   }

NOTE

Is code splitting worth it? You need to splitting at least dozens of kilobytes for code-splitting to be actually worth it.

Lazy loading only when component is visible

For the best performance gains, you could load a component only when it is about to visible in the DOM.

There are two methods you could use for this:

• Use the react-intersection-observer library
• Use the react-loadable-visibility library

react-intersection-observer

We can combine the intersection observer along with <Suspense> and React.lazy() to lazy load components and show them only when they are in view, boosting our performance.

1. Install with npm i react-intersection-observer
2. Use the useInView() hook to load a component only when it is in view.

import { Suspense, lazy } from "react";
import { useInView } from "react-intersection-observer";
const Listing = lazy(() => import("./components/Listing"));

function ListingCard(props) {
  const { ref, inView } = useInView();

  return (
    <div ref={ref}>
      <Suspense fallback={<div />}>{inView && <Listing />}</Suspense>
    </div>
  );
}

If using React Router, you can enable code splitting for routes like so:

import React, { lazy, Suspense } from "react";
import { Switch, Route, BrowserRouter as Router } from "react-router-dom";

const App = lazy(() => import("./App"));
const About = lazy(() => import("./About"));
const Contact = lazy(() => import("./Contact"));

ReactDOM.render(
  <Router>
    <Suspense fallback={<div>Loading...</div>}>
      <Switch>
        <Route exact path="/">
          <App />
        </Route>
        <Route path="/about">
          <About />
        </Route>
        <Route path="/contact">
          <Contact />
        </Route>
      </Switch>
    </Suspense>
  </Router>,
  document.getElementById("root")
);

react-loadable-visibility

import LoadableVisibility from "react-loadable-visibility/react-loadable";
import Loading from "./my-loading-component";
 
const LoadableComponent = LoadableVisibility({
  loader: () => import("./my-component"),
  loading: Loading
});
 
export default function App() {
  return <LoadableComponent />;
}

State-lifting vs colocation

There are two ways we provide state to components in React.

The default behavior of React is to rerender the entire component tree if any component within that tree has a state change. If we lift state too much, then the component tree becomes larger and larger and thus the app not only rerenders more often, but renders also become more expensive.

We should prefer colocation to state lifting whenever possible.

Memoization in React: main performance boost

useMemo() and useCallback() and memo()

In React, JSX elements only rerender once their props change. To check if their props changed, they use a referential equality check ===, which works for primitives but not for objects or functions.

NOTE

This means that if you create a function or object inside a component and then pass that as a prop to a JSX element, then that function or object gets recreated every single time and thus the props strict equality check does not work because the references are different, leading to the JSX element rerendering every time if you pass an object or a callback as a prop.

The solution? Memoizing objects with useMemo() and functions with useCallback()

• useMemo(cb, deps): takes in a callback and a dependency array, returning the return value of the callback.
  • The callback should return a value, and that value will be memoized, only changing when at least one of the variables in the dependency array changes.
• useCallback(cb, deps): takes in a callback and a dependency array, returns the callback as a memoized version of it.
  • The callback will only get invoked again if any of the dependencies in the dependencies array changes.
  • This returns the callback itself.

The dependency arrays check if any dependency variables changed by strict equality (by reference with ===).

This means that for both of the dependency arrays of useMemo() and useCallback(), the variables you pass into the dependency array must either be primitive values or memoized.

• bad scenario: If you pass unmemoized objects and functions as variables into the dependency array, then those objects/functions are getting recreated every time, thus have different references and thus useMemo() and useCallback() fails and just recreate the object/function each and every time.

We can fix this bad scenario by adhering to two good techniques:

• mitigation 1 (use primitive dependencies only): Instead of putting entire objects as variables into the dependency array, put object properties that are primitive values.
  • Primitive values work with strict equality.
• mitigation 2 (memoize objects and callbacks): Go up the chain and use useMemo() and useCallback() to memoize any objects or functions you put inside the dependency array. This leads to referential equality working.

Here's an example of mitigation 1:

function App({person} : {person : {name: string; age: number}}) {
	const greeting = useMemo(() => {
		return `Hello ${person.name}. You are ${person.age} years old`
	}, [person.name, person.age])
}

Here's an example of mitigation 2:

function App({person} : {person : {name: string; age: number}}) {

	// memoized version of person
	const newPerson = useMemo(() => {
		return person
	}, [person.name, person.age])
	
	// now it's safe to use newPerson as dependency, as it only changes
	// when Object.is(newPerson, person) is false
	const greeting = useMemo(() => {
		return `Hello ${newPerson.name}. You are ${newPerson.age} years old`
	}, [newPerson])
}

NOTE

You should use useMemo() primarily for memoizing objects instead of primitive values. This is because there is no need to memoize primitive values (since they pass strict equality checks). Only memoize primitive values if they are derived from an expensive calculation that should be cached for better performance.

IMPORTANT

An also important thing to remember is that any setState callback is automatically memoized, so you don't need to pass that function into the dependency array as that function always has the same reference.

Now if you pass objects memoized with useMemo() and callbacks memoized with useCallback() as props to JSX elements, then that JSX element will not rerender unless the variables in the dependency arrays change.

However, it's a different story for components. Because React likes to err on the side of caution, components always rerender any time 1) a parent component rerenders 2) a state change in the component occurs.

We use the memo() function to wrap a component such that the component will not rerender unless its props change from the last render. However, the props are compared with strict equality, so we must use useMemo() and useCallback() to also memoize any object/function props passed to the component, in tandem with wrapping with memo().

In summary, if you are passing an object or function as a prop:

• to prevent JSX Element rerender: simply memoize that object or function with useMemo() or useCallback()
• to prevent component rerender: You must wrap the component with memo() and then memoize the object or function props with useMemo() or useCallback()

useMemo vs useCallback

The main difference between useMemo() and useCallback() is that useMemo() should be used for objects while useCallback() should be used to explicitly cache functions.

However, useCallback() is just useMemo() but returning a function instead of a object - it's essentially just syntactic sugar.

when to use each one

• use useMemo() when:
  • case 1: trying to get a constant or predictable reference to an object so it only gets recreated when one of its dependencies changes.
  • case 2: memoizing an expensive calculation result so it doesn't recompute on every rerender.
• use useCallback() when
  • case 1: needing to memoize a callback so it passes referential equality, only being recreated when one of its dependencies changes.

Context

Understanding how to use context is important for improving performance in React applications:

When a component uses some value from context, and the context value changes, the component will re-render.

Based off that, here are two important tips to implement:

1. A good rule of thumb is to wrap the component you directly nest inside the context provider with React.memo().
2. Memoize any objects/functions/primitives that you provide to the value prop in the <Context.Provider> component, either using useMemo() or useCallback() to prevent unnecessary recreations.

Transitions and deferred values

useTransition()

For anything we would want to debounce, like a search bar that performs an expensive search operation for every keystroke, we want to use the useTransition() hook.

Often what happens for something like an expensive search query is that a user will typ a keystroke, initiating a laggy response, and then find that it's clunky and janky since the user input is disabled while the long search operation is taking place.

This hooks solves that problem. The purpose of this hook is to keep the UI and user interactions snappy by setting low priority work (the search operation) to happen later when the DOM is not budy, so you can tackle the high priority work (setting keystroke state to keep snappy user interactions ) instead.

NOTE

What marks high priority work? It's anything that deals with immediate DOM manipulation that is visible to the user, or DOM manipulation created by some user interaction.

Reflecting user input back to them is high priority because if the UI starts lagging and freezing, preventing users from seeing the results of their actions (like typing, hovering, or animations), it creates an infuriating user experience. Users expect immediate feedback from their interactions.

function App() {
	const [isPending, startTransition] = useTransition()
	
	// state that handles input user interaction, high priority
	const [input, setInput] = useState("")
	// state that is logical value of input, low prioirity, expensive work
	const [query, setQuery] = useState("")
	
	
	function onKeystroke(value) {
		// high piroity work: immediately reflect user interaction in DOM
		setInput(value)
		
		// low priority work: set state that will then kickoff expensive operation
		startTransition(() => {
			setQuery(value)
		})
	}
	
	useEffect(() => {
		// perform expensive operation in idle time
		expensiveSearchOperation(query)
	}, [query])
	
	
	
}

Here are two cases when NOT to use transitions:

• state changes are user-visible and important: navigations, error validation, alerts, are all high priority DOM manipulation work.
• state changes are simple and inexpensive: Transitioning any simple state change that doesn't kick off any expensive operation or giant component tree rerender just causes additional overhead that doesn't solve anything.

Here's another example:

import { useTransition, useState } from 'react';

// ✅ Smooth typing with transitions
function SearchResults() {
  const [query, setQuery] = useState('');
  const [filteredQuery, setFilteredQuery] = useState('');
  const [items] = useState(generateLargeDataset());
  const [isPending, startTransition] = useTransition();

  const filteredItems = items.filter((item) =>
    item.name.toLowerCase().includes(filteredQuery.toLowerCase()),
  );

  const handleSearch = (value: string) => {
    setQuery(value); // Urgent: update input immediately

    startTransition(() => {
      setFilteredQuery(value); // Non-urgent: defer filtering
    });
  };

  return (
    <div>
      <input value={query} onChange={(e) => handleSearch(e.target.value)} placeholder="Search..." />
      {isPending && <div>Filtering...</div>}
      {filteredItems.map((item) => (
        <div key={item.id}>{item.name}</div>
      ))}
    </div>
  );
}

• Immediate update: setQuery(value) runs synchronously, and this sets the state of the controlled input, handling DOM manipulation and updates keystrokes so that they immediately visible to the user and so typing stays responsive
• Deferred update: setFilteredQuery(value) inside startTransition gets lower priority, since we run expensive computation based on the value of that state3.
• Loading state: isPending tells us when the transition is still processing, basically isPending will be true as long as the DOM is busy (doing a setState() call somewhere).
  • Think of it as automatic debouncing. It will be pending until the DOM sotps being busy (the user stops typing).

Here is an example of combining debouncing with a transition

function AdvancedSearch() {
  const [query, setQuery] = useState('');
  const [results, setResults] = useState([]);
  const [isPending, startTransition] = useTransition();

  // Debounce the search to avoid excessive API calls
  const debouncedSearch = useCallback(
    debounce((searchQuery: string) => {
      startTransition(() => {
        // This could be an API call or expensive filtering
        performSearch(searchQuery).then(setResults);
      });
    }, 300),
    [],
  );

  useEffect(() => {
    if (query) {
      debouncedSearch(query);
    } else {
      setResults([]);
    }
  }, [query, debouncedSearch]);

  return (
    <div>
      <input
        value={query}
        onChange={(e) => setQuery(e.target.value)}
        placeholder="Search products..."
      />

      {isPending && <div>Searching...</div>}

      <div className="results">
        {results.map((result) => (
          <SearchResult key={result.id} item={result} />
        ))}
      </div>
    </div>
  );
}

There are two common problems to look out for, and tips on how to mitigate them:

Stale Closures in Transitions

Be careful with closures inside startTransition:

// ❌ This captures stale values
const handleUpdate = () => {
  startTransition(() => {
    // `someValue` might be stale if the transition is interrupted
    setResults(processData(someValue));
  });
};

// ✅ Get fresh values inside the transition
const handleUpdate = () => {
  startTransition(() => {
    setResults((currentResults) => processData(getCurrentValue()));
  });
};

Transitions Don’t Make Code Faster

Transitions don’t magically speed up your code—they just prevent slow code from blocking urgent updates:

// ❌ Still slow, just non-blocking
startTransition(() => {
  setResults(reallySlowOperation(data)); // This is still slow!
});

// ✅ Combine with other optimizations
startTransition(() => {
  // Use web workers, memoization, virtualization, etc.
  setResults(optimizedOperation(data));
});

useDeferredValue()

useDeferredValue() defers state changes that trigger expensive computation, delegating them to be run during browser idle time.

import { useDeferredValue, useState, useMemo } from 'react';

function SearchResults({ query }: { query: string }) {
  // Defer state change based on prop `query` changing.
  const deferredQuery = useDeferredValue(query);

  // Only recompute when the deferred value changes
  const results = useMemo(() => {
    return searchExpensiveDatabase(deferredQuery);
  }, [deferredQuery]);

  return (
    <div>
      {results.map((result) => (
        <div key={result.id}>{result.title}</div>
      ))}
    </div>
  );
}

function App() {
  const [query, setQuery] = useState('');

  return (
    <div>
      <input
        value={query}
        onChange={(e) => setQuery(e.target.value)}
        placeholder="Search products..."
      />
      <SearchResults query={query} />
    </div>
  );
}

IMPORTANT

Always pair useDeferredValue with useMemo or useCallback—otherwise, your components will still re-render on every change, defeating the purpose.

However, using useDeferredValue() leads to stale values while the transition is taking place, as opposed to the isPending variable from the useTransition() hook.

To mitigate this, we can simply say that the loading state happens when the data is stale, and that happens when the deferred value is not equal to the normal state value.

function SearchWithLoadingState() {
  const [query, setQuery] = useState('');
  const deferredQuery = useDeferredValue(query);

  const results = useMemo(() => {
    // Simulate expensive search
    return performExpensiveSearch(deferredQuery);
  }, [deferredQuery]);

  // Key insight: results are "stale" when the current query
  // doesn't match the deferred query
  const isStale = query !== deferredQuery;

  return (
    <div>
      <input value={query} onChange={(e) => setQuery(e.target.value)} placeholder="Search..." />

      <div className="results-container">
        {isStale && <div className="loading-overlay">Searching...</div>}
        <div className={isStale ? 'results stale' : 'results'}>
          {results.map((result) => (
            <div key={result.id}>{result.title}</div>
          ))}
        </div>
      </div>
    </div>
  );
}

Here are some examples of when to use this hook and when not to use it:

// ❌ Don't defer critical user feedback
function LoginForm() {
  const [email, setEmail] = useState('');
  const deferredEmail = useDeferredValue(email); // Bad idea!

  const validationErrors = useMemo(() => {
    return validateEmail(deferredEmail);
  }, [deferredEmail]);

  // User expects immediate validation feedback
  return (
    <div>
      <input value={email} onChange={(e) => setEmail(e.target.value)} />
      {validationErrors.map((error) => (
        <div key={error}>{error}</div>
      ))}
    </div>
  );
}

// ❌ Don't defer simple computations
function SimpleCounter() {
  const [count, setCount] = useState(0);
  const deferredCount = useDeferredValue(count); // Unnecessary overhead

  return <div>Count: {deferredCount}</div>;
}

// ✅ DO use it for expensive, non-critical updates
function AnalyticsDashboard() {
  const [query, setQuery] = useState('');
  const deferredQuery = useDeferredValue(query);

  const expensiveStats = useMemo(() => {
    return calculateComplexAnalytics(deferredQuery);
  }, [deferredQuery]);

  return (
    <div>
      <input value={query} onChange={(e) => setQuery(e.target.value)} />
      <ComplexChart data={expensiveStats} />
    </div>
  );
}

Here are some common gotchas:

Gotcha #1: Missing Memoization

// ❌ Still re-renders on every change
function BadExample({ query }: { query: string }) {
  const deferredQuery = useDeferredValue(query);

  // This runs on every render!
  const results = expensiveComputation(deferredQuery);

  return <div>{results}</div>;
}

// ✅ Properly memoized
function GoodExample({ query }: { query: string }) {
  const deferredQuery = useDeferredValue(query);

  const results = useMemo(() => {
    return expensiveComputation(deferredQuery);
  }, [deferredQuery]);

  return <div>{results}</div>;
}

Gotcha #2: Deferring the Wrong Thing

// ❌ Deferring the final result instead of the input
function BadExample({ items, query }: { items: Item[]; query: string }) {
  const filteredItems = items.filter((item) => item.name.includes(query));
  const deferredItems = useDeferredValue(filteredItems); // Wrong!

  return <ItemList items={deferredItems} />;
}

// ✅ Defer the input, memoize the computation
function GoodExample({ items, query }: { items: Item[]; query: string }) {
  const deferredQuery = useDeferredValue(query);

  const filteredItems = useMemo(() => {
    return items.filter((item) => item.name.includes(deferredQuery));
  }, [items, deferredQuery]);

  return <ItemList items={filteredItems} />;
}

Transitions vs deferred values

• useTransition is used when you control the code and allows you to pass a function that marks certain operations as low priority. 
  • this is more common
• useDeferredValue is used when you don't control the code and works with a value that may trigger expensive operations, allowing you to defer updates to that value.
  • this is uncommon

NOTE

You should reach for useTransition first when you control the code. If that doesn't fit due to constraints where you only have access to a value and don't have total control of the situation, then use useDeferredValue.

Using virtualized lists

JUst like <FlatList> component in react native, we can virtualize lists of elements to make them more performant. In react we do this through the react-window library from npm:

import React from "react";
import ReactDOM from "react-dom";
import { FixedSizeList as List } from "react-window";

const itemsArray = [...]; // our data

const Row = ({ index, style }) => (
  <div className={index % 2 ? "ListItemOdd" : "ListItemEven"} style={style}>
    {itemsArray[index].name}
  </div>
);

const Example = () => (
  <List
    className="List"
    height={150}
    itemCount={itemsArray.length}
    itemSize={35}
    width={300}
  >
    {Row}
  </List>
);

ReactDOM.render(<Example />, document.getElementById("root"));

React query

Setup

Here is how to setup react query:

import {
  QueryClient,
  QueryClientProvider,
} from '@tanstack/react-query';
import { ReactQueryDevtools } from '@tanstack/react-query-devtools';

function App() {

  return (
    <QueryClientProvider client={queryClient}>
        <BrowserRouter>
	        <Routes>
	          <Route path="/" element={<Index />} />
	          {/* more routes */}
	        </Routes>
	    </BrowserRouter>
      {import.meta.env.DEV && <ReactQueryDevtools />}
    </QueryClientProvider>
  )
}

export default App

useQuery

Here is a basic example of how to use react query:

• queryKey: a string array that will be an ID to be used for caching
• queryFn: an async function that returns something, which is the actual querying.
• staleTime: how long to cache for in milliseconds, infinite by default.
• retry: how many times to retry. By default, it's 0.
• refetchInterval: the interval of time to wait before refetching again, in milliseconds.
• enabled: lets you conditionally execute the hook by passing a boolean to enable or disable the hook. If true, enables the hook (default), if false, disables the hook.

function FlightSearchResults() {
  const {
    data: flights,
    isLoading,
    error,
  } = useQuery({
    queryKey: ['flights', flightSearch], // ✅ Automatic caching by key
    queryFn: () => fetchFlights(flightSearch), // ✅ Simple data fetching
    staleTime: 5 * 60 * 1000, // ✅ Cache for 5 minutes
    retry: 2, // ✅ Automatic retry on failure
  });

  // ✅ Same conditional rendering, but managed automatically
  if (isLoading) return <div>Loading...</div>;
  if (error) return <div>Error: {error.message}</div>;
  return <div>{/* render flights */}</div>;
}

// src/components/PostDetail.jsx
import React from 'react';
import { useQuery } from '@tanstack/react-query';

const fetchPostById = async (postId) => {
  const response = await fetch(`https://jsonplaceholder.typicode.com/posts/${postId}`);
  if (!response.ok) {
    throw new Error('Failed to fetch post');
  }
  return response.json();
};

function PostDetail({ postId }) {
  const { data, isLoading, isError, error } = useQuery({
    // Query key includes postId, so changing postId will refetch
    queryKey: ['post', postId],
    // queryFn receives a query context object, from which you can destructure queryKey
    queryFn: ({ queryKey }) => fetchPostById(queryKey[1]), // queryKey[1] is postId
    // `enabled` option: only run this query if `postId` is truthy
    enabled: !!postId,
  });

  if (!postId) {
    return <div>Select a post to view details.</div>;
  }

  if (isLoading) {
    return <div>Loading post {postId}...</div>;
  }

  if (isError) {
    return <div>Error loading post: {error.message}</div>;
  }

  return (
    <div>
      <h2>{data.title}</h2>
      <p>{data.body}</p>
    </div>
  );
}

export default PostDetail;

useMutation

To create a mutation with the useMutation hook, you pass in these options to the useMutation() hook:

• mutationFn: an async function that performs some mutation, like a POST request, and can also return data. It can accept any number of arguments, since it's a function you create.
• onMutate(data): a callback that gets triggered during mutation, intended to be used for optimistic updates. The data arg provided is whatever was returned from the async mutation fn.

You also have these optional, less important callback you can pass in to hook into the mutation lifecycle.

• onSuccess(data, args): callback that gets triggered when the async mutation function successfully executed.
  • data: the data that gets returned from the mutation function.
  • args: the arguments that were passed into the create mutation
• onError(err, args): callback that gets triggered when the async mutation function fails and throws an error.
  • err: the error thrown
  • args: the arguments that were passed into the create mutation
• onSettled(data, err args): callback that gets triggered when the async mutation function promise settles.
  • data: the data that gets returned from the mutation function.
  • err: the error thrown
  • args: the arguments that were passed into the create mutation

// src/components/CreatePostForm.jsx
import React, { useState } from 'react';
import { useMutation, useQueryClient } from '@tanstack/react-query';

const createPost = async (newPost) => {
  const response = await fetch('https://jsonplaceholder.typicode.com/posts', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify(newPost),
  });
  if (!response.ok) {
    throw new Error('Failed to create post');
  }
  return response.json();
};

function CreatePostForm() {
  const queryClient = useQueryClient();
  const [title, setTitle] = useState('');
  const [body, setBody] = useState('');

  const {
    mutate,       // The function to trigger the mutation
    isLoading,    // True while the mutation is in progress
    isError,      // True if the mutation failed
    isSuccess,    // True if the mutation succeeded
    error         // Error object
  } = useMutation({
    mutationFn: createPost,
    onSuccess: (data, variables, context) => {
      // data: the response from createPost
      // variables: the `newPost` object passed to mutate
      // context: optional context object from onMutate

      console.log('Post created successfully:', data);

      // --- Invalidation and Refetching ---
      // Invalidate the 'posts' query to trigger a refetch of the list
      queryClient.invalidateQueries({ queryKey: ['posts'] });

      // Optionally, you could also directly update the cache:
      // queryClient.setQueryData(['posts'], oldPosts => {
      //   return oldPosts ? [...oldPosts, data] : [data];
      // });
    },
    onError: (error, variables, context) => {
      console.error('Error creating post:', error);
      // Rollback optimistic update if any
    },
    onSettled: (data, error, variables, context) => {
      // Runs regardless of success or error
      console.log('Mutation settled.');
    },
    // --- Optimistic Updates ---
    // Optimistically update the UI BEFORE the server responds
    // onMutate is called before the mutationFn
    onMutate: async (newPost) => {
      // Cancel any outgoing refetches for the posts list (important!)
      await queryClient.cancelQueries({ queryKey: ['posts'] });

      // Snapshot the current cached posts list (for potential rollback)
      const previousPosts = queryClient.getQueryData(['posts']);

      // Optimistically update the 'posts' cache
      queryClient.setQueryData(['posts'], oldPosts => {
        // Generate a temporary ID for the new post
        const tempId = Math.random().toString(36).substring(7);
        const optimisticPost = { id: tempId, ...newPost, userId: 1 }; // Add any default server-side props

        return oldPosts ? [...oldPosts, optimisticPost] : [optimisticPost];
      });

      // Return a context object with the snapshot
      return { previousPosts };
    },
    // If the mutation fails, use the context to roll back the cache
    onError: (err, newPost, context) => {
      queryClient.setQueryData(['posts'], context.previousPosts);
    },
  });

  const handleSubmit = (e) => {
    e.preventDefault();
    mutate({ title, body }); // Trigger the mutation
    setTitle('');
    setBody('');
  };

  return (
    <div>
      <h2>Create New Post</h2>
      <form onSubmit={handleSubmit}>
        <div>
          <label htmlFor="title">Title:</label>
          <input
            id="title"
            type="text"
            value={title}
            onChange={(e) => setTitle(e.target.value)}
            disabled={isLoading}
          />
        </div>
        <div>
          <label htmlFor="body">Body:</label>
          <textarea
            id="body"
            value={body}
            onChange={(e) => setBody(e.target.value)}
            disabled={isLoading}
          ></textarea>
        </div>
        <button type="submit" disabled={isLoading}>
          {isLoading ? 'Creating...' : 'Create Post'}
        </button>
      </form>
      {isSuccess && <p style={{ color: 'green' }}>Post created!</p>}
      {isError && <p style={{ color: 'red' }}>Error: {error.message}</p>}
    </div>
  );
}

export default CreatePostForm;

queryClient

You can access the query client through the useQueryClient() hook, which allows you to ahve global access to all data querying and mutation operations throughout your app, and programmatically refetch and invalidate them at will:

  const queryClient = useQueryClient();

• queryClient.invalidateQueries({ queryKey: key }): Marks queries matching the key as stale. They will refetch when next observed or on global refetch triggers (focus, reconnect).
• queryClient.refetchQueries({ queryKey: key }): Forces an immediate refetch of queries matching the key, regardless of their stale status.
• queryClient.cancelQueries({ queryKey: key }): cancels all queries regarding the filter.

Here are single-query specific things you can do, doing something with a query attached to a specific key.

• queryClient.getQueryData(key): gets the query data associated with the specified key.
• queryClient.setQueryData(key, data): sets the query data associated with the specified key to some new data.

// Invalidate all queries
queryClient.invalidateQueries();

// Invalidate all queries starting with 'posts'
queryClient.invalidateQueries({ queryKey: ['posts'] });

// Invalidate specific query
queryClient.invalidateQueries({ queryKey: ['post', 123] });

// Invalidate all queries for a specific type and status
queryClient.invalidateQueries({ queryKey: ['todos'], exact: false }); // All todos
queryClient.invalidateQueries({ queryKey: ['todos', { status: 'pending' }], exact: true }); // Only pending todos

// Force refetch all
queryClient.refetchQueries();

Pagination

// src/components/PaginatedPosts.jsx
import React, { useState } from 'react';
import { useQuery } from '@tanstack/react-query';

const fetchPaginatedPosts = async (page) => {
  const response = await fetch(`https://jsonplaceholder.typicode.com/posts?_page=${page}&_limit=10`);
  if (!response.ok) throw new Error('Failed to fetch paginated posts');
  return response.json();
};

function PaginatedPosts() {
  const [page, setPage] = useState(1);

  const {
    data,
    isLoading,
    isError,
    error,
    isPreviousData // True if data from previous query key is being shown
  } = useQuery({
    queryKey: ['paginatedPosts', page], // Query key changes with page number
    queryFn: ({ queryKey }) => fetchPaginatedPosts(queryKey[1]),
    keepPreviousData: true, // Keep old data visible while fetching new
  });

  if (isLoading) return <div>Loading posts...</div>;
  if (isError) return <div>Error: {error.message}</div>;

  return (
    <div>
      <h1>Paginated Posts (Page {page})</h1>
      <ul>
        {data.map(post => (
          <li key={post.id}>
            <h3>{post.title}</h3>
            <p>{post.body.substring(0, 100)}...</p>
          </li>
        ))}
      </ul>
      <div>
        <button
          onClick={() => setPage(old => Math.max(old - 1, 1))}
          disabled={page === 1}
        >
          Previous Page
        </button>
        <button
          onClick={() => {
            // Placeholder for knowing if there's a next page.
            // In a real app, API would return total pages/count.
            // For JSONPlaceholder, we assume there are more pages until 100
            if (data.length === 10) { // If we got 10 items, there might be more
              setPage(old => old + 1);
            }
          }}
          disabled={isPreviousData || data.length < 10} // Disable if previous data is shown or no more items
        >
          Next Page
        </button>
        {isPreviousData && <p>Loading next page in background...</p>}
      </div>
    </div>
  );
}

export default PaginatedPosts;

Infinite scrolling

// src/components/InfinitePosts.jsx
import React from 'react';
import { useInfiniteQuery } from '@tanstack/react-query';

const fetchInfinitePosts = async ({ pageParam = 1 }) => {
  const response = await fetch(`https://jsonplaceholder.typicode.com/posts?_page=${pageParam}&_limit=10`);
  if (!response.ok) throw new Error('Failed to fetch infinite posts');
  return response.json();
};

function InfinitePosts() {
  const {
    data,         // Contains pages: [{ data: [...], ... }, { data: [...], ... }]
    fetchNextPage, // Function to load the next page
    hasNextPage,  // True if there's a next page to fetch
    isFetchingNextPage, // True while fetching the next page
    isLoading,
    isError,
    error,
  } = useInfiniteQuery({
    queryKey: ['infinitePosts'],
    queryFn: fetchInfinitePosts,
    getNextPageParam: (lastPage, allPages) => {
      // lastPage: The data for the last fetched page
      // allPages: An array of all fetched pages so far
      // Return the pageParam for the next fetch, or undefined if no more pages
      const nextPage = allPages.length + 1;
      // For JSONPlaceholder, max posts are 100, so max 10 pages
      if (nextPage <= 10) {
        return nextPage;
      }
      return undefined;
    },
    // Optional: initialData for immediate display from SSR or pre-existing cache
    // initialData: { pages: [initialPosts], pageParams: [1] },
  });

  if (isLoading) return <div>Loading posts...</div>;
  if (isError) return <div>Error: {error.message}</div>;

  return (
    <div>
      <h1>Infinite Posts</h1>
      {data.pages.map((page, i) => (
        <React.Fragment key={i}>
          {page.map(post => (
            <div key={post.id} style={{ borderBottom: '1px solid #ccc', padding: '10px 0' }}>
              <h3>{post.title}</h3>
              <p>{post.body}</p>
            </div>
          ))}
        </React.Fragment>
      ))}
      <button
        onClick={() => fetchNextPage()}
        disabled={!hasNextPage || isFetchingNextPage}
      >
        {isFetchingNextPage
          ? 'Loading more...'
          : hasNextPage
          ? 'Load More'
          : 'Nothing more to load'}
      </button>
    </div>
  );
}

export default InfinitePosts;

React router

Basic browser router

Scroll to top

In newer versions of react router 7+, you can just render the <ScrollRestoration /> component from react-router-dom at the top level of the app, and that automatically saves scroll position.

If you want to force scrolling to the top each time, then you should use the useLocation() hook to scroll up the page every time the URL changes, and make a custom hook from this:

import { useLayoutEffect } from "react";
import { useLocation } from "react-router-dom";

export function useScrollToTop() {
  const { pathname } = useLocation();

  useLayoutEffect(() => {
    window.scrollTo(0, 0);
  }, [pathname]);

  return null;
}

Then you use this hook somewhere in your app tree:

const ScrollRestorer = () => {
  useScrollToTop();
  return null;
};

const App = () => (
        <BrowserRouter>
          <ScrollRestorer />
          <Routes>
            <Route path="/" element={<Index />} />
            <Route path="/faq" element={<FAQ />} />
            <Route path="/contact" element={<Contact />} />
            <Route path="/order" element={<Order />} />
            <Route path="/order-result" element={<OrderResult />} />
            <Route path="/order-history" element={<OrderHistory />} />
            <Route path="/orders/:id" element={<CustomerOrderDetailPage />} />
            <Route path="/privacy-policy" element={<PrivacyPolicy />} />
            <Route path="/terms-of-service" element={<TermsOfService />} />
            <Route path="/admin" element={<AdminPage />} />
            <Route path="/admin/manage-orders" element={<AdminPage />} />
            <Route path="/admin/orders/:id" element={<OrderDetailPage />} />
            <Route path="*" element={<NotFound />} />
          </Routes>
        </BrowserRouter>
);

export default App;

React 19 changes

The React 19 changes remove a lot of annoying optimization code we once had to write ourselves, like useMemo() or useCallback() or memo(). This is because the react compiler looks ahead of time at the correct optimizations to make. But besides this main feature, there are new changes:

removal of forwardRef()

If you want to pass a ref prop to a custom child component, you needed to wrap it in a forwardRef() call. Well, not anymore. All you need to do know is to destructure the ref prop in the child component props and type annotate it accordingly.

use() hook

The use() hook is a way to asynchronously deal with data fetching and other things that take time without having to use a useEffect() call.

async function delay(ms: number) {
  return new Promise((resolve) => setTimeout(resolve, ms));
}

async function fetchData() {
  await delay(1000);
  return "Hello, world!";
}

const UseExample = () => {
  const message = use(fetchData());
  return <div>{message}</div>;
};

const App = () => {
  return (
    <div>
      <Suspense fallback={<div>Loading...</div>}>
        <UseExample />
      </Suspense>
    </div>
  );
};

import { use } from 'react'
 
export default function Posts({
  posts,
}: {
  posts: Promise<{ id: string; title: string }[]>
}) {
  const allPosts = use(posts)
 
  return (
    <ul>
      {allPosts.map((post) => (
        <li key={post.id}>{post.title}</li>
      ))}
    </ul>
  )
}

useTransition()

The useTransition() hook allows developers to mark state updates as transitions. This can help keep the UI responsive by treating certain updates as non-urgent. Updates marked as transitions won't block urgent updates, such as typing in an input field.`

The useTransition() hook is used to keep the UI interactive while doing some asynchronous operation. It yields to the main thread while ensuring the final state change is correct.

export function Example() {
	const [isPending, startTransition] = useTransition()
}

• isPending: a boolean value representing the loading state
• startTransition(): a function that takes in an async callback of some code you want to run. This will defer to the main thread while still running the callback.

example

In a tabbed interface, switching tabs might fetch new data for each tab. Using useTransition ensures that the UI remains responsive, even if the data fetch is slow. The user can switch tabs without delay, and data for each tab loads without causing the UI to freeze.

import { useState, useTransition } from 'react';

function TabContainer() {
  const [isPending, startTransition] = useTransition();
  const [tab, setTab] = useState('home');

  function selectTab(nextTab) {
    startTransition(() => {
      setTab(nextTab);
    });
  }

  return (
    // Tab UI components here
  );
}

key points

• Non-blocking updates: Marking an update as a transition prevents it from blocking interaction with the UI.
• Handling multiple transitions: Currently, React batches multiple transitions, although this may change in future releases.
• Limitations: Transitions are not suitable for controlling text inputs and must be synchronous.

Optimistic UI udpates: useOptimistic()

An optimistic uI update is the UI updating to reflect the result of some data fetching or long-running asynchronous action before the data action actually completes

This can lead to the UI feeling snappy, but if the server action fails, you have to rollback the ui updates to reflect the true result of the server action. This can be confusing for a user who was led to believe that the server action succeeded because of the optimistic UI update.

To use this hook you need two existing pieces of code:

• transitions: You need to nest any optimistic update code inside a startTransition() invocation from the useTransition() hook, since optimistic updates are always low-priority operations.
• standard state: You need to pass in a standard state to the useOptimistic() hook so it can perform automatic rollbacks to that state.

export function Example() {
	const [isPending, startTransition] = useTransition()
	const [notes, setNotes] = useState<Note[]>([])
	const [optimisticNotes, addOptimisticNote] = useOptimistic(
		notes, 
		(oldNotes, newNote) => {
			// if pending, optimistically show new note
			if (newNote.isPending) {
				return [...oldNotes, newNote]
			}
			// if not pending, roll back to old version
			return oldNotes.filter(n -> n.id !== newNote.id)
		}
	)
}

• The first argument we pass to the useOptimistic() hook is the base state, which will be the initial value of the optimisticNotes state and also the rollback value for when the true state changes with setNotes()
• The second argument we pass to the useOptimistic() hook is a mutation callback where we set the state
• The isPending property allows the UI to differentiate between optimistic (temporary) items and real items that have been confirmed by the server. This can be used to apply visual styling, such as a faded state, to indicate to the user that the item is still being saved and hasn't been confirmed yet.

useDeferredValue()

In react, everything is marked a high priority render by default. If you want to change expensive computational state changes to low priority renders, then you have to explicitly mark them as low priority with the useOptimistic() nad useDeferredValue() hooks.

The useDeferredValue() hook specifically is used for stuff like debouncing and throttling, where it only assumes the most recent state change after a rapid succession of state changes. It takes in a state variable as an argument:

const [state, setState] = useState()
const deferredState = useDefrredValue(state)

You can then create a "debouncing" effect by preventing action when the original state is not equal to the deferred value:

const isUpdating = state !== deferredState

useEffectEvent()

The useEffectEvent() hook creates a method that can be used in useEffect but doesn't need to be added to the dependency array.

1. useEffectEvent(cb) takes in a callback and returns the callback, but no need for the dependency array.
2. You can then use this callback inside a useEffect and no need to pass in that callback to the effect's dependency array.

import { useEffect, useEffectEvent } from 'react';

function Page({ url }) {
  const { cartItems } = useContext(ShoppingCartContext);

  // This is an Effect Event: it ALWAYS sees the latest cartItems
  // but it is NOT a dependency for the Effect.
  const onVisit = useEffectEvent((visitedUrl) => {
    logAnalytics('visit', {
      url: visitedUrl,
      numberOfItems: cartItems.length // Fresh data, but non-reactive
    });
  });

  useEffect(() => {
    onVisit(url);
    // ✅ The Effect only re-runs when the 'url' changes.
    // Changes to 'cartItems' do NOT trigger a new log.
  }, [url]); 
}

useActionState()

The useActionState() hook in react 19 is also available in nextJS and is used as a better way of handling forms, falling back to the OG way: using actions.

However, in client side react, we can replace the onSubmit= handler with the action= prop, and pass in an async function called a handler that passes the FormData gathered from the form submission. This means we can use this hook in our client side code and get rid of controlled inputs, which cause unnecessary re-renders.

async function submitForm(formData: FormData) {
  const rawData = Object.fromEntries(formData);
  
  const travelFormSchema = z.object({
	  firstName: z.string().min(1, 'First name is required'),
	  lastName: z.string().min(1, 'Last name is required'),
	  email: z.string().email('Invalid email address'),
	  age: z.coerce.number().min(18, 'Must be 18 or older'),
	});

  // Validate with Zod
  const result = travelFormSchema.safeParse(rawData);

  if (!result.success) {
    return {
      status: 'error',
      errors: result.error.flatten().fieldErrors,
    };
  }

  // Now we have type-safe, validated data
  const validData = result.data;
  // perform some mutation or a fetch request ...
  
  return {
	  error: null,
	  status: "success"
  }
}

const initialState = {
	error: null,
	status: "none"
}

export function Form() {
	const [state, submitAction, isPending] = useActionState(
	  serverAction,
	  initialState
	);

	return (
	  <form action={submitAction}>
	    {/* isPending gives you loading state */}
	    {/* state contains response/errors */}
	  </form>
	);
}

Use FormData when:

• Building traditional forms with submit buttons
• Working with server actions/mutations
• Need progressive enhancement
• Forms have many fields
• File uploads are involved
• Working with Next.js app router

Use useState when:

• Building real-time/interactive UIs
• Need immediate validation on every keystroke
• Complex client-side logic between fields
• Building search/filter interfaces
• Need granular control over individual field updates
• Working with controlled components that need precise state

SSR from scratch in React

Step 1) typescript needs to be happy

---

In your tsconfig.json or deno,json, make sure that React gets the appropriate syntax highlighting through these compiler options:

tsconfig.json

"compilerOptions": {
	"jsx": "react-jsx",
	"jsxImportSource": "react"
}

Step 2) create React App entry point

---

Create a react app, standard:

import React from "react";
// import { JSX } from "npm:react/jsx-runtime";

const App = () => {
  return (
    <div>
      <h1>Hello World</h1>
    </div>
  );
};

export default App;

Step 3: render on the server

Using the react-dom/server package from npm, we can render react to HTML in two different ways:

• rendering to static markup: Strips away all javascript and returns just the static HTML formed from the react pp.
• rendering with hydration: Uses hydration to add in javascript once app is statically rendered.

The react-dom/server package exposes two methods that let us tap into rendering either way:

import { renderToString, renderToStaticMarkup } from "react-dom/server";
import App from "./frontend/App.tsx";

const hydratedHTML = renderToString(App());
const staticHTML = renderToStaticMarkup(App());

• renderToString(component): Renders the react app with hydration
• renderToStaticMarkup(component): Renders the react app completely statically.

This is how you would do completely static rendering:

import { DenoRouter } from "./DenoRouter.ts";
import { renderToStaticMarkup } from "react-dom/server";
import App from "./frontend/App.tsx";
const router = new DenoRouter();

router.get("/", (req, res) => {
  const html = renderToStaticMarkup(App());
  return router.renderHTML(html);
});

router.initServer(8000);

Hydration

Hydration is a bit more difficult since you need to explicitly create a separate javascript client that gets loaded in and does the hydration (making the page interactive, which lets stuff like hooks work) for us.

The thing that hydrates our app is on the client side, the hydrateRoot() method from react dom. Below is the main javascript file we want to separately bundle as a javascript file that we then serve in the HTML:

client.ts

import { hydrateRoot } from "react-dom/client";
import App from "./App.tsx";
import React from "react";

hydrateRoot(document.getElementById("root")!, React.createElement(App));

Then on the server, we'll do two important things:

1. Make the client hydration JS file statically available on our server, serve it statically.
2. Render the react app and enable the possibility of hydration with renderToString(), then serve it as HTML and referencing the client JS script to fetch

import { DenoRouter } from "./DenoRouter.ts";
import { renderToString } from "react-dom/server";
import React, { createElement } from "react";
import App from "./frontend/App.tsx";
import { Intellisense } from "./is.ts";
import path from "node:path";

const router = new DenoRouter();

const __filename = new URL(import.meta.url).pathname;
const __dirname = path.dirname(__filename);

// Serve the client-side bundle
router.get("/client.js", async (req, res) => {
  const clientCode = `
import React from "https://esm.sh/react@19.1.0";
import { hydrateRoot } from "https://esm.sh/react-dom@19.1.0/client";

const App = () => {
  const [count, setCount] = React.useState(0);
  return React.createElement("div", null,
    React.createElement("h1", null, "Hello World"),
    React.createElement("button", { onClick: () => setCount(count + 1) }, "Click me"),
    React.createElement("p", null, "Count: ", count)
  );
};

hydrateRoot(document.getElementById("root"), React.createElement(App));
  `;

  return new Response(clientCode, {
    headers: { "Content-Type": "application/javascript" },
  });
});

router.get("/", (req, res) => {
// 1. render static HTML
  const html = renderToString(React.createElement(App));
  return router.renderHTML(
    Intellisense.html`
      <head>
        <title>Hello World</title>
      </head>
      <body>
        <div id="root">${html}</div>
        <! -- reference the client side bundle that does hydration -->
        <script type="module" async defer src="/client.js"></script>
      </body>
    `
  );
});

router.initServer(8000);

RSCs from scratch: react flight protocol

The main difference between RSCs and SSR is that RSCs are just react components that only run on the server. SSR does the initial rendering on the server, and then client side rendering through hydration takes over.

With RSCs, there is no client side rendering nor hydration. However, you can nest client components within server components.

The main way RSCs get translated to actual HTML is through the react flight protocol, which is an HTTP request with content type application/octet-stream that is basically a JSON representation of the JSX to render.

const MANIFEST = readFileSync(
  path.resolve(__dirname, "../dist/react-client-manifest.json"),
  "utf8"
);
const MODULE_MAP = JSON.parse(MANIFEST);

fastify.get("/react-flight", function reactFlightHandler(request, reply) {
  try {
    reply.header("Content-Type", "application/octet-stream");
    const { pipe } = renderToPipeableStream(
      React.createElement(App),
      MODULE_MAP
    );
    pipe(reply.raw);
  } catch (err) {
    request.log.error("react-flight err", err);
    throw err;
  }
});

Zustand

Zustand vs Context

Zustand is basically the same thing as context, just an improvement on top of it.

When using react context, you wrap your entire app in the context provider. If even a single one of the exported state in the context changes, then the context provider re-renders, causing the entire app to re-render and any components that use the context to re-render (even if memoized).

Let's walk through what happens if you use context:

1. if either theme or setTheme change, then the context provider will re-render
2. Every child component of the context provider will thus also re-render
3. Even if memoized, if a child component uses context via the useContext() hook, it will re-render, even if doesn't use a part of context that updated. It doesn't matter, since one state change updates the entire context object and thus a state update is registered.

// this component will re-render if theme or setTheme change
function GrandChildComponent() {
  const { theme, setTheme } = useContext(ThemeContext)

  return (
    <>
      <div>The theme is {theme}</div>
    </>
  )

Zustand is a more performant solution here because it only forces a child component to re-render when the specific part of context it uses updates state. This leads to less unnecessary re-renders.

import {create} from "zustand"

interface CounterStore {
	count: number;
	increment: () => number;
	decrement: () => number;
	reset: () => void;
}

export const useCounterStore = create<CounterStore>(
	(set) => ({
		count: 0;
		increment: () => {
			// automatically merges objects
			set(state => ({count: state.count + 1})) 
		},
		decrement: () => {
			set(state => ({count: state.count - 1})) 
		},
		reset: () => {
			set({ count: 0})
		}
	})
)

Zustand out of the box works the exact same as context, with no performance benefits, as shown in this example:

function CounterExample() {
	// we grab the whole object, so re-renders when a single property
	// of the object changes.
	const { increment } = useCounterStore()
	
	return (
		<button onClick={increment}>increment</button>
	)
}

Here's why there's no performance benefit in the above example:

• The zustand store hook returns an entire object. Even if we destructure only the properties we need, we're still grabbing the entire object first.
• This means if a single property in the store changes do to a state update, like count even though we don't render or use count anywhere in the component, it changes the entire store object (since store.count changes), so a state udpate and thus re-render takes place.

NOTE

A store change in zustand is registered when at least one property of the store object has had a state update or change.

What we need to do instead is only grab the variables we need to avoid the unnecessary state-updates for a component, which we can do via this syntax, called store slicing syntax

function CounterExample() {
	// only grab the store.increment function
	const increment = useCounterStore(store => store.increment)
	
	return (
		<button onClick={increment}>increment</button>
	)
}

In store slicing, how does zustand know when increment has changed and re-render accordingly? In this example, we return store.increment, so increment changes when store.increment changes somehow.

NOTE

Zustand uses value-equality for primitives and referential-equality for objects, checking if the single variable we are grabbing has changed in a store change.

This means if we use store slicing syntax and return a new object, using referential equality, that is a new object each and every time thus we get shoved into an infinite loop.

function CounterExample() {
	// ❌ common trap: this actually kicks you into an infinite loop
	const {increment, decrement} = useCounterStore(store => ({
			increment: store.increment,
			decrement: store.decrement
		})
	)
	
	return (
		<button onClick={increment}>increment</button>
	)
}

However, for convenience sake, if you want to destructure more than one property at once from the store without losing the performance benefits of store slicing, then use the useShallow() hook from zustand to wrap that object so that any object returned from store slicing is now checked with value equality rather than referential equality, stringifying the object to compare its values across re-renders:

import {useShallow} from "zustand"

function CounterExample() {
	// ✅ now no infinite loop
	const {increment, decrement} = useCounterStore(store => useShallow({
			increment: store.increment,
			decrement: store.decrement
		})
	)
	
	return (
		<button onClick={increment}>increment</button>
	)
}

Zustand is signal-based

Zustand is signal-based. That means you can use it outside react components.

Every zustand store hook has these two methods

• useStore.getState(): returns the current state of the store as an object
• useStore.setState(cb): sets the state of the store, accepting a callback that has one argument - the current state of the store.

Zustand best practices

Refactor state-setters into actions

State-setter functions in a store are called actions, and for readability and ease of use, actions should be markedly separate from state, which we can achieve by nesting all actions inside of an actions object.

Here are the benefits:

1. cleaner code
2. less re-renders: extracting our actions out of the store means the store has less reason to change, avoiding unnecessary re-renders.

import {create} from "zustand"

interface CounterStore {
	count: number;
}

interface CounterStoreActions {
	increment: () => number;
	decrement: () => number;
	reset: () => void;
}

export const useCounterStore = create<CounterStore>(
	(set) => ({
		count: 0;
	})
)

export const counterStoreActions = {
	increment: () => {
			// automatically merges objects
			useCounterStore.setState(state => ({count: state.count + 1})) 
	},
	decrement: () => {
		useCounterStore.setState(state => ({count: state.count - 1})) 
	},
	reset: () => {
		useCounterStore.setState({ count: 0 })
	}
}

Custom Hooks

useWaitForAnimationsToFinish

• main idea: given an HTML element ref, we want to wait for until all of its CSS animations have finished.
• execution: Await all CSS animation status promises of the ref, store the finished state in a ref (not state because we don't use that to update the view)

useNetworkStatus

import * as React from "react"

const getSnapshot = () => {
  return navigator.onLine ? "online" : "offline"
}

const subscribe = (callback) => {
  window.addEventListener("online", callback)
  window.addEventListener("offline", callback)

  return () => {
    window.removeEventListener("online", callback)
    window.removeEventListener("offline", callback)
  }
}

export default function useNetworkStatus () {
  const networkStatus = return React.useSyncExternalStore(
    subscribe, 
    getSnapshot
  )
  return networkStatus
}

useInterval

function useInterval(cb, ms) {
  const id = React.useRef(null);
  const onInterval = React.useEffectEvent(cb);

  const handleClearInterval = () => {
    window.clearInterval(id.current);
  };

  React.useEffect(() => {
    id.current = window.setInterval(onInterval, ms);
    return handleClearInterval;
  }, [ms]);

  return handleClearInterval;
}

Custom components

Boop

The "Boop" animation is basically based on hover state. On a mouse enter animation, we toggle some state and turn it off after a certain timeout.

import React from "react";

const useBoop = (timing: number = 150) => {
  const [isBooped, setIsBooped] = React.useState(false);
  React.useEffect(() => {
    if (!isBooped) {
      return;
    }
    // reset the state after the timeout
    const timeoutId = setTimeout(() => {
      setIsBooped(false);
    }, timing);
    return () => {
      clearTimeout(timeoutId);
    };
  }, [isBooped, timing]);

  const trigger = () => {
    setIsBooped(true);
  };

  return { trigger, isBooped };
};

export default useBoop;

Then here is a component to wrap any element and give it the Boop effect:

const Boop = ({ rotation = 20, timing = 150, children }) => {
  const { isBooped, trigger } = useBoop(timing);
  const style = {
    display: "inline-block",
    transform: isBooped ? `rotate(${rotation}deg)` : `rotate(0deg)`,
    transition: `transform ${timing}ms`,
  };
  return (
    <span onMouseEnter={trigger} style={style}>
      {children}
    </span>
  );
};