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React Compiler is a new build-time tool that automatically optimizes your React app. It works with plain JavaScript, and understands theRules of React, so you don’t need to rewrite any code to use it.

You will learn

  • What React Compiler does
  • Getting started with the compiler
  • Incremental adoption strategies
  • Debugging and troubleshooting when things go wrong
  • Using the compiler on your React library

What does React Compiler do?

React Compiler automatically optimizes your React application at build time. React is often fast enough without optimization, but sometimes you need to manually memoize components and values to keep your app responsive. This manual memoization is tedious, easy to get wrong, and adds extra code to maintain. React Compiler does this optimization automatically for you, freeing you from this mental burden so you can focus on building features.

Before React Compiler

Without the compiler, you need to manually memoize components and values to optimize re-renders:

import{useMemo,useCallback,memo}from'react';

constExpensiveComponent =memo(functionExpensiveComponent({data,onClick}){
constprocessedData =useMemo(()=>{
returnexpensiveProcessing(data);
},[data]);

consthandleClick =useCallback((item)=>{
onClick(item.id);
},[onClick]);

return(
<div>
{processedData.map(item=>(
<Itemkey={item.id}onClick={()=>handleClick(item)}/>
))}
</div>
);
});

Note

This manual memoization has a subtle bug that breaks memoization:

<Itemkey={item.id}onClick={() => handleClick(item)}/>

Even thoughhandleClick is wrapped inuseCallback, the arrow function() => handleClick(item) creates a new function every time the component renders. This means thatItem will always receive a newonClick prop, breaking memoization.

React Compiler is able to optimize this correctly with or without the arrow function, ensuring thatItem only re-renders whenprops.onClick changes.

After React Compiler

With React Compiler, you write the same code without manual memoization:

functionExpensiveComponent({data,onClick}){
constprocessedData =expensiveProcessing(data);

consthandleClick =(item)=>{
onClick(item.id);
};

return(
<div>
{processedData.map(item=>(
<Itemkey={item.id}onClick={()=>handleClick(item)}/>
))}
</div>
);
}

See this example in the React Compiler Playground

React Compiler automatically applies the optimal memoization, ensuring your app only re-renders when necessary.

Deep Dive

What kind of memoization does React Compiler add?

React Compiler’s automatic memoization is primarily focused onimproving update performance (re-rendering existing components), so it focuses on these two use cases:

  1. Skipping cascading re-rendering of components
    • Re-rendering<Parent /> causes many components in its component tree to re-render, even though only<Parent /> has changed
  2. Skipping expensive calculations from outside of React
    • For example, callingexpensivelyProcessAReallyLargeArrayOfObjects() inside of your component or hook that needs that data

Optimizing Re-renders

React lets you express your UI as a function of their current state (more concretely: their props, state, and context). In its current implementation, when a component’s state changes, React will re-render that componentand all of its children — unless you have applied some form of manual memoization withuseMemo(),useCallback(), orReact.memo(). For example, in the following example,<MessageButton> will re-render whenever<FriendList>’s state changes:

functionFriendList({friends}){
constonlineCount =useFriendOnlineCount();
if(friends.length ===0){
return<NoFriends/>;
}
return(
<div>
<span>{onlineCount} online</span>
{friends.map((friend)=>(
<FriendListCardkey={friend.id}friend={friend}/>
))}
<MessageButton/>
</div>
);
}

See this example in the React Compiler Playground

React Compiler automatically applies the equivalent of manual memoization, ensuring that only the relevant parts of an app re-render as state changes, which is sometimes referred to as “fine-grained reactivity”. In the above example, React Compiler determines that the return value of<FriendListCard /> can be reused even asfriends changes, and can avoid recreating this JSXand avoid re-rendering<MessageButton> as the count changes.

Expensive calculations also get memoized

React Compiler can also automatically memoize expensive calculations used during rendering:

// **Not** memoized by React Compiler, since this is not a component or hook
functionexpensivelyProcessAReallyLargeArrayOfObjects(){/* ... */}

// Memoized by React Compiler since this is a component
functionTableContainer({items}){
// This function call would be memoized:
constdata =expensivelyProcessAReallyLargeArrayOfObjects(items);
// ...
}

See this example in the React Compiler Playground

However, ifexpensivelyProcessAReallyLargeArrayOfObjects is truly an expensive function, you may want to consider implementing its own memoization outside of React, because:

  • React Compiler only memoizes React components and hooks, not every function
  • React Compiler’s memoization is not shared across multiple components or hooks

So ifexpensivelyProcessAReallyLargeArrayOfObjects was used in many different components, even if the same exact items were passed down, that expensive calculation would be run repeatedly. We recommendprofiling first to see if it really is that expensive before making code more complicated.

Should I try out the compiler?

We encourage everyone to start using React Compiler. While the compiler is still an optional addition to React today, in the future some features may require the compiler in order to fully work.

Is it safe to use?

React Compiler is now stable and has been tested extensively in production. While it has been used in production at companies like Meta, rolling out the compiler to production for your app will depend on the health of your codebase and how well you’ve followed theRules of React.

What build tools are supported?

React Compiler can be installed acrossseveral build tools such as Babel, Vite, Metro, and Rsbuild.

React Compiler is primarily a light Babel plugin wrapper around the core compiler, which was designed to be decoupled from Babel itself. While the initial stable version of the compiler will remain primarily a Babel plugin, we are working with the swc andoxc teams to build first class support for React Compiler so you won’t have to add Babel back to your build pipelines in the future.

Next.js users can enable the swc-invoked React Compiler by usingv15.3.1 and up.

What should I do about useMemo, useCallback, and React.memo?

By default, React Compiler will memoize your code based on its analysis and heuristics. In most cases, this memoization will be as precise, or moreso, than what you may have written.

However, in some cases developers may need more control over memoization. TheuseMemo anduseCallback hooks can continue to be used with React Compiler as an escape hatch to provide control over which values are memoized. A common use-case for this is if a memoized value is used as an effect dependency, in order to ensure that an effect does not fire repeatedly even when its dependencies do not meaningfully change.

For new code, we recommend relying on the compiler for memoization and usinguseMemo/useCallback where needed to achieve precise control.

For existing code, we recommend either leaving existing memoization in place (removing it can change compilation output) or carefully testing before removing the memoization.

Try React Compiler

This section will help you get started with React Compiler and understand how to use it effectively in your projects.

Additional resources

In addition to these docs, we recommend checking theReact Compiler Working Group for additional information and discussion about the compiler.

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