useCallback

const cachedFn = useCallback(fn, dependencies)

• Usage
  • Skipping re-rendering of components
  • Updating state from a memoized callback
  • Preventing an Effect from firing too often
  • Optimizing a custom Hook
• Reference
  • useCallback(fn, dependencies)
• Troubleshooting
  • Every time my component renders, useCallback returns a different function
  • I need to call useCallback for each list item in a loop, but it’s not allowed

Usage

Skipping re-rendering of components

When you optimize rendering performance, you will sometimes need to cache the functions that you pass to child components. Let’s first look at the syntax for how to do this, and then see in which cases it’s useful.

To cache a function between re-renders of your component, wrap its definition into the useCallback Hook:

import { useCallback } from 'react';

function ProductPage({ productId, referrer, theme }) {
  const handleSubmit = useCallback((orderDetails) => {
    post('/product/' + productId + '/buy', {
      referrer,
      orderDetails,
    });
  }, [productId, referrer]);
  // ...

You need to pass two things to useCallback:

1. A function definition that you want to cache between re-renders.
2. A list of dependencies including every value within your component that’s used inside your function.

On the initial render, the returned function you’ll get from useCallback will be the function you passed.

On the following renders, React will compare the dependencies with the dependencies you passed during the previous render. If none of the dependencies have changed (compared with Object.is), useCallback will return the same function as before. Otherwise, useCallback will return the function you passed on this render.

In other words, useCallback caches a function between re-renders until its dependencies change.

Let’s walk through an example to see when this is useful.

Say you’re passing a handleSubmit function down from the ProductPage to the ShippingForm component:

function ProductPage({ productId, referrer, theme }) {
  // ...
  return (
    <div className={theme}>
      <ShippingForm onSubmit={handleSubmit} />
    </div>
  );

You’ve noticed that toggling the theme prop freezes the app for a moment, but if you remove <ShippingForm /> from your JSX, it feels fast. This tells you that it’s worth trying to optimize the ShippingForm component.

By default, when a component re-renders, React re-renders all of its children recursively. This is why, when ProductPage re-renders with a different theme, the ShippingForm component also re-renders. This is fine for components that don’t require much calculation to re-render. But if you’ve verified that a re-render is slow, you can tell ShippingForm to skip re-rendering when its props are the same as on last render by wrapping it in memo:

import { memo } from 'react';

const ShippingForm = memo(function ShippingForm({ onSubmit }) {
  // ...
});

With this change, ShippingForm will skip re-rendering if all of its props are the same as on the last render. This is where caching a function becomes important! Imagine that you defined handleSubmit without useCallback:

function ProductPage({ productId, referrer, theme }) {
  // Every time the theme changes, this will be a different function...
  function handleSubmit(orderDetails) {
    post('/product/' + productId + '/buy', {
      referrer,
      orderDetails,
    });
  }
  
  return (
    <div className={theme}>
      {/* ... so ShippingForm's props will never be the same, and it will re-render every time */}
      <ShippingForm onSubmit={handleSubmit} />
    </div>
  );
}

In JavaScript, a function () {} or () => {} always creates a different function, similar to how the {} object literal always creates a new object. Normally, this wouldn’t be a problem, but it means that ShippingForm props will never be the same, and your memo optimization won’t work. This is where useCallback comes in handy:

function ProductPage({ productId, referrer, theme }) {
  // Tell React to cache your function between re-renders...
  const handleSubmit = useCallback((orderDetails) => {
    post('/product/' + productId + '/buy', {
      referrer,
      orderDetails,
    });
  }, [productId, referrer]); // ...so as long as these dependencies don't change...

  return (
    <div className={theme}>
      {/* ...ShippingForm will receive the same props and can skip re-rendering */}
      <ShippingForm onSubmit={handleSubmit} />
    </div>
  );
}

By wrapping handleSubmit in useCallback, you ensure that it’s the same function between the re-renders (until dependencies change). You don’t have to wrap a function in useCallback unless you do it for some specific reason. In this example, the reason is that you pass it to a component wrapped in memo, and this lets it skip re-rendering. There are a few other reasons you might need useCallback which are described further on this page.

import { useMemo, useCallback } from 'react';

function ProductPage({ productId, referrer }) {
  const product = useData('/product/' + productId);

  const requirements = useMemo(() => { // Calls your function and caches its result
    return computeRequirements(product);
  }, [product]);

  const handleSubmit = useCallback((orderDetails) => { // Caches your function itself
    post('/product/' + productId + '/buy', {
      referrer,
      orderDetails,
    });
  }, [productId, referrer]);

  return (
    <div className={theme}>
      <ShippingForm requirements={requirements} onSubmit={handleSubmit} />
    </div>
  );
}

// Simplified implementation (inside React)
function useCallback(fn, dependencies) {
  return useMemo(() => fn, dependencies);
}

Updating state from a memoized callback

Sometimes, you might need to update state based on previous state from a memoized callback.

This handleAddTodo function specifies todos as a dependency because it computes the next todos from it:

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

  const handleAddTodo = useCallback((text) => {
    const newTodo = { id: nextId++, text };
    setTodos([...todos, newTodo]);
  }, [todos]);
  // ...

You’ll usually want your memoized functions to have as few dependencies as possible. When you read some state only to calculate the next state, you can remove that dependency by passing an updater function instead:

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

  const handleAddTodo = useCallback((text) => {
    const newTodo = { id: nextId++, text };
    setTodos(todos => [...todos, newTodo]);
  }, []); // ✅ No need for the todos dependency
  // ...

Here, instead of making todos a dependency of your function and reading it there, you pass an instruction about how to update the state (todos => [...todos, newTodo]) to React. Read more about updater functions.

Preventing an Effect from firing too often

Sometimes, you might want to call a function from inside an Effect:

function ChatRoom({ roomId }) {
  const [message, setMessage] = useState('');

  function createOptions() {
    return {
      serverUrl: 'https://localhost:1234',
      roomId: roomId
    };
  }

  useEffect(() => {
    const options = createOptions();
    const connection = createConnection();
    connection.connect();
    // ...

This creates a problem. Every reactive value must be declared as a dependency of your Effect. However, if you declare createOptions as a dependency, it will cause your Effect to constantly reconnect to the chat room:

  useEffect(() => {
    const options = createOptions();
    const connection = createConnection();
    connection.connect();
    return () => connection.disconnect();
  }, [createOptions]); // 🔴 Problem: This dependency changes on every render
  // ...

To solve this, you can wrap the function you need to call from an Effect into useCallback:

function ChatRoom({ roomId }) {
  const [message, setMessage] = useState('');

  const createOptions = useCallback(() => {
    return {
      serverUrl: 'https://localhost:1234',
      roomId: roomId
    };
  }, [roomId]); // ✅ Only changes when roomId changes

  useEffect(() => {
    const options = createOptions();
    const connection = createConnection();
    connection.connect();
    return () => connection.disconnect();
  }, [createOptions]); // ✅ Only changes when createOptions changes
  // ...

This ensures that the createOptions function is the same between re-renders if the roomId is the same. However, it’s even better to remove the need for a function dependency. Move your function inside the Effect:

function ChatRoom({ roomId }) {
  const [message, setMessage] = useState('');

  useEffect(() => {
    function createOptions() { // ✅ No need for useCallback or function dependencies!
      return {
        serverUrl: 'https://localhost:1234',
        roomId: roomId
      };
    }

    const options = createOptions();
    const connection = createConnection();
    connection.connect();
    return () => connection.disconnect();
  }, [roomId]); // ✅ Only changes when roomId changes
  // ...

Now your code is simpler and doesn’t need useCallback. Learn more about removing Effect dependencies.

Optimizing a custom Hook

If you’re writing a custom Hook, it’s recommended to wrap any functions that it returns into useCallback:

function useRouter() {
  const { dispatch } = useContext(RouterStateContext);

  const navigate = useCallback((url) => {
    dispatch({ type: 'navigate', url });
  }, [dispatch]);

  const goBack = useCallback(() => {
    dispatch({ type: 'back' });
  }, [dispatch]);

  return {
    navigate,
    goBack,
  };
}

This ensures that the consumers of your Hook can optimize their own code when needed.

Reference

useCallback(fn, dependencies)

Call useCallback at the top level of your component to declare a memoized callback:

import { useCallback } from 'react';

export default function ProductPage({ productId, referrer, theme }) {
  const handleSubmit = useCallback((orderDetails) => {
    post('/product/' + productId + '/buy', {
      referrer,
      orderDetails,
    });
  }, [productId, referrer]);

See more examples above.

Parameters

• fn: The function value that you want to memoize. It can take any arguments and return any values. React will return (not call!) your function back to you during the initial render. On subsequent renders, React will return the same function again if the dependencies have not changed since the last render. Otherwise, it will give you the function that you have passed during the current render, and store it in case it can be reused later. React will not call the function. The function is returned to you so you can decide when and whether to call it.

• dependencies: The list of all reactive values referenced inside of the fn code. Reactive values include props, state, and all the variables and functions declared directly inside your component body. If your linter is configured for React, it will verify that every reactive value is correctly specified as a dependency. The list of dependencies must have a constant number of items and be written inline like [dep1, dep2, dep3]. React will compare each dependency with its previous value using the Object.is comparison algorithm.

Returns

On the initial render, useCallback returns the fn function you have passed.

During subsequent renders, it will either return an already stored fn function from the last render (if the dependencies haven’t changed), or return the fn function you have passed during this render.

Caveats

• useCallback is a Hook, so you can only call it at the top level of your component or your own Hooks. You can’t call it inside loops or conditions. If you need that, extract a new component and move the state into it.
• React will not throw away the cached function unless there is a specific reason to do that. For example, in development, React throws away the cache when you edit the file of your component. Both in development and in production, React will throw away the cache if your component suspends during the initial mount. In the future, React may add more features that take advantage of throwing away the cache—for example, if React adds built-in support for virtualized lists in the future, it would make sense to throw away the cache for items that scroll out of the virtualized table viewport. This should match your expectations if you rely on useCallback as a performance optimization. Otherwise, a state variable or a ref may be more appropriate.

Troubleshooting

Every time my component renders, useCallback returns a different function

Make sure you’ve specified the dependency array as a second argument!

If you forget the dependency array, useCallback will return a new function every time:

function ProductPage({ productId, referrer }) {
  const handleSubmit = useCallback((orderDetails) => {
    post('/product/' + productId + '/buy', {
      referrer,
      orderDetails,
    });
  }); // 🔴 Returns a new function every time: no dependency array
  // ...

This is the corrected version passing the dependency array as a second argument:

function ProductPage({ productId, referrer }) {
  const handleSubmit = useCallback((orderDetails) => {
    post('/product/' + productId + '/buy', {
      referrer,
      orderDetails,
    });
  }, [productId, referrer]); // ✅ Does not return a new function unnecessarily
  // ...

If this doesn’t help, then the problem is that at least one of your dependencies is different from the previous render. You can debug this problem by manually logging your dependencies to the console:

  const handleSubmit = useCallback((orderDetails) => {
    // ..
  }, [productId, referrer]);

  console.log([productId, referrer]);

You can then right-click on the arrays from different re-renders in the console and select “Store as a global variable” for both of them. Assuming the first one got saved as temp1 and the second one got saved as temp2, you can then use the browser console to check whether each dependency in both arrays is the same:

Object.is(temp1[0], temp2[0]); // Is the first dependency the same between the arrays?
Object.is(temp1[1], temp2[1]); // Is the second dependency the same between the arrays?
Object.is(temp1[2], temp2[2]); // ... and so on for every dependency ...

When you find which dependency is breaking memoization, either find a way to remove it, or memoize it as well.

I need to call useCallback for each list item in a loop, but it’s not allowed

Suppose the Chart component is wrapped in memo. You want to skip re-rendering every Chart in the list when the ReportList component re-renders. However, you can’t call useCallback in a loop:

function ReportList({ items }) {
  return (
    <article>
      {items.map(item => {
        // 🔴 You can't call useCallback in a loop like this:
        const handleClick = useCallback(() => {
          sendReport(item)
        }, [item]);

        return (
          <figure key={item.id}>
            <Chart onClick={handleClick} />
          </figure>
        );
      })}
    </article>
  );
}

Instead, extract a component for an individual item, and put useCallback there:

function ReportList({ items }) {
  return (
    <article>
      {items.map(item =>
        <Report key={item.id} item={item} />
      )}
    </article>
  );
}

function Report({ item }) {
  // ✅ Call useCallback at the top level:
  const handleClick = useCallback(() => {
    sendReport(item)
  }, [item]);

  return (
    <figure>
      <Chart onClick={handleClick} />
    </figure>
  );
}

Alternatively, you could remove useCallback in the last snippet and instead wrap Report itself in memo. If the item prop does not change, Report will skip re-rendering, so Chart will skip re-rendering too:

function ReportList({ items }) {
  // ...
}

const Report = memo(function Report({ item }) {
  function handleClick() {
    sendReport(item);
  }

  return (
    <figure>
      <Chart data={data} />
    </figure>
  );
});