Hooks are a new addition in React 16.8. They let you use state and other React features without writing a class.

The introduction page used this example to get familiar with Hooks:

import React, { useState } from 'react';

function Example() {
  // Declare a new state variable, which we'll call "count"  const [count, setCount] = useState(0);
  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>
        Click me
      </button>
    </div>
  );
}

We’ll start learning about Hooks by comparing this code to an equivalent class example.

Equivalent Class Example

If you used classes in React before, this code should look familiar:

class Example extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      count: 0
    };
  }

  render() {
    return (
      <div>
        <p>You clicked {this.state.count} times</p>
        <button onClick={() => this.setState({ count: this.state.count + 1 })}>
          Click me
        </button>
      </div>
    );
  }
}

The state starts as { count: 0 } , and we increment state.count when the user clicks a button by calling this.setState() . We’ll use snippets from this class throughout the page.

Note

You might be wondering why we’re using a counter here instead of a more realistic example. This is to help us focus on the API while we’re still making our first steps with Hooks.

Hooks and Function Components

As a reminder, function components in React look like this:

const Example = (props) => {
  // You can use Hooks here!
  return <div />;
}

or this:

function Example(props) {
  // You can use Hooks here!
  return <div />;
}

You might have previously known these as “stateless components”. We’re now introducing the ability to use React state from these, so we prefer the name “function components”.

Hooks don’t work inside classes. But you can use them instead of writing classes.

What’s a Hook?

Our new example starts by importing the useState Hook from React:

import React, { useState } from 'react';
function Example() {
  // ...
}

What is a Hook? A Hook is a special function that lets you “hook into” React features. For example, useState is a Hook that lets you add React state to function components. We’ll learn other Hooks later.

When would I use a Hook? If you write a function component and realize you need to add some state to it, previously you had to convert it to a class. Now you can use a Hook inside the existing function component. We’re going to do that right now!

Note:

There are some special rules about where you can and can’t use Hooks within a component. We’ll learn them in Rules of Hooks.

Declaring a State Variable

In a class, we initialize the count state to 0 by setting this.state to { count: 0 } in the constructor:

class Example extends React.Component {
  constructor(props) {
    super(props);
    this.state = {      count: 0    };  }

In a function component, we have no this , so we can’t assign or read this.state . Instead, we call the useState Hook directly inside our component:

import React, { useState } from 'react';

function Example() {
  // Declare a new state variable, which we'll call "count"  const [count, setCount] = useState(0);

What does calling useState do? It declares a “state variable”. Our variable is called count but we could call it anything else, like banana . This is a way to “preserve” some values between the function calls — useState is a new way to use the exact same capabilities that this.state provides in a class. Normally, variables “disappear” when the function exits but state variables are preserved by React.

What do we pass to useState as an argument? The only argument to the useState() Hook is the initial state. Unlike with classes, the state doesn’t have to be an object. We can keep a number or a string if that’s all we need. In our example, we just want a number for how many times the user clicked, so pass 0 as initial state for our variable. (If we wanted to store two different values in state, we would call useState() twice.)

What does useState return? It returns a pair of values: the current state and a function that updates it. This is why we write const [count, setCount] = useState() . This is similar to this.state.count and this.setState in a class, except you get them in a pair. If you’re not familiar with the syntax we used, we’ll come back to it at the bottom of this page.

Now that we know what the useState Hook does, our example should make more sense:

import React, { useState } from 'react';

function Example() {
  // Declare a new state variable, which we'll call "count"  const [count, setCount] = useState(0);

We declare a state variable called count , and set it to 0 . React will remember its current value between re-renders, and provide the most recent one to our function. If we want to update the current count , we can call setCount .

Note

You might be wondering: why is useState not named createState instead?

“Create” wouldn’t be quite accurate because the state is only created the first time our component renders. During the next renders, useState gives us the current state. Otherwise it wouldn’t be “state” at all! There’s also a reason why Hook names always start with use . We’ll learn why later in the Rules of Hooks.

Reading State

When we want to display the current count in a class, we read this.state.count :

  <p>You clicked {this.state.count} times</p>

In a function, we can use count directly:

  <p>You clicked {count} times</p>

Updating State

In a class, we need to call this.setState() to update the count state:

  <button onClick={() => this.setState({ count: this.state.count + 1 })}>    Click me
  </button>

In a function, we already have setCount and count as variables so we don’t need this :

  <button onClick={() => setCount(count + 1)}>    Click me
  </button>

Recap

Let’s now recap what we learned line by line and check our understanding.

 1:  import React, { useState } from 'react'; 2:
 3:  function Example() {
 4:    const [count, setCount] = useState(0); 5:
 6:    return (
 7:      <div>
 8:        <p>You clicked {count} times</p>
 9:        <button onClick={() => setCount(count + 1)}>10:         Click me
11:        </button>
12:      </div>
13:    );
14:  }

• Line 1: We import the useState Hook from React. It lets us keep local state in a function component.


• Line 4: Inside the Example component, we declare a new state variable by calling the useState Hook. It returns a pair of values, to which we give names. We’re calling our variable count because it holds the number of button clicks. We initialize it to zero by passing 0 as the only useState argument. The second returned item is itself a function. It lets us update the count so we’ll name it setCount .


• Line 9: When the user clicks, we call setCount with a new value. React will then re-render the Example component, passing the new count value to it.

This might seem like a lot to take in at first. Don’t rush it! If you’re lost in the explanation, look at the code above again and try to read it from top to bottom. We promise that once you try to “forget” how state works in classes, and look at this code with fresh eyes, it will make sense.

Tip: What Do Square Brackets Mean?

You might have noticed the square brackets when we declare a state variable:

  const [count, setCount] = useState(0);

The names on the left aren’t a part of the React API. You can name your own state variables:

  const [fruit, setFruit] = useState('banana');

This JavaScript syntax is called “array destructuring”. It means that we’re making two new variables fruit and setFruit , where fruit is set to the first value returned by useState , and setFruit is the second. It is equivalent to this code:

  var fruitStateVariable = useState('banana'); // Returns a pair
  var fruit = fruitStateVariable[0]; // First item in a pair
  var setFruit = fruitStateVariable[1]; // Second item in a pair

When we declare a state variable with useState , it returns a pair — an array with two items. The first item is the current value, and the second is a function that lets us update it. Using [0] and [1] to access them is a bit confusing because they have a specific meaning. This is why we use array destructuring instead.

Note

You might be curious how React knows which component useState corresponds to since we’re not passing anything like this back to React. We’ll answer this question and many others in the FAQ section.

Tip: Using Multiple State Variables

Declaring state variables as a pair of [something, setSomething] is also handy because it lets us give different names to different state variables if we want to use more than one:

function ExampleWithManyStates() {
  // Declare multiple state variables!
  const [age, setAge] = useState(42);
  const [fruit, setFruit] = useState('banana');
  const [todos, setTodos] = useState([{ text: 'Learn Hooks' }]);

In the above component, we have age , fruit , and todos as local variables, and we can update them individually:

  function handleOrangeClick() {
    // Similar to this.setState({ fruit: 'orange' })
    setFruit('orange');
  }

You don’t have to use many state variables. State variables can hold objects and arrays just fine, so you can still group related data together. However, unlike this.setState in a class, updating a state variable always replaces it instead of merging it.

We provide more recommendations on splitting independent state variables in the FAQ.

Next Steps

On this page we’ve learned about one of the Hooks provided by React, called useState . We’re also sometimes going to refer to it as the “State Hook”. It lets us add local state to React function components — which we did for the first time ever!

We also learned a little bit more about what Hooks are. Hooks are functions that let you “hook into” React features from function components. Their names always start with use , and there are more Hooks we haven’t seen yet.

Now let’s continue by learning the next Hook: useEffect . It lets you perform side effects in components, and is similar to lifecycle methods in classes.

Hooks are a new addition in React 16.8. They let you use state and other React features without writing a class.

The Effect Hook lets you perform side effects in function components:

import React, { useState, useEffect } from 'react';
function Example() {
  const [count, setCount] = useState(0);

  // Similar to componentDidMount and componentDidUpdate:  useEffect(() => {    // Update the document title using the browser API    document.title = `You clicked ${count} times`;  });
  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>
        Click me
      </button>
    </div>
  );
}

This snippet is based on the counter example from the previous page, but we added a new feature to it: we set the document title to a custom message including the number of clicks.

Data fetching, setting up a subscription, and manually changing the DOM in React components are all examples of side effects. Whether or not you’re used to calling these operations “side effects” (or just “effects”), you’ve likely performed them in your components before.

Tip

If you’re familiar with React class lifecycle methods, you can think of useEffect Hook as componentDidMount , componentDidUpdate , and componentWillUnmount combined.

There are two common kinds of side effects in React components: those that don’t require cleanup, and those that do. Let’s look at this distinction in more detail.

Effects Without Cleanup

Sometimes, we want to run some additional code after React has updated the DOM. Network requests, manual DOM mutations, and logging are common examples of effects that don’t require a cleanup. We say that because we can run them and immediately forget about them. Let’s compare how classes and Hooks let us express such side effects.

Example Using Classes

In React class components, the render method itself shouldn’t cause side effects. It would be too early — we typically want to perform our effects after React has updated the DOM.

This is why in React classes, we put side effects into componentDidMount and componentDidUpdate . Coming back to our example, here is a React counter class component that updates the document title right after React makes changes to the DOM:

class Example extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      count: 0
    };
  }

  componentDidMount() {    document.title = `You clicked ${this.state.count} times`;  }  componentDidUpdate() {    document.title = `You clicked ${this.state.count} times`;  }
  render() {
    return (
      <div>
        <p>You clicked {this.state.count} times</p>
        <button onClick={() => this.setState({ count: this.state.count + 1 })}>
          Click me
        </button>
      </div>
    );
  }
}

Note how we have to duplicate the code between these two lifecycle methods in class.

This is because in many cases we want to perform the same side effect regardless of whether the component just mounted, or if it has been updated. Conceptually, we want it to happen after every render — but React class components don’t have a method like this. We could extract a separate method but we would still have to call it in two places.

Now let’s see how we can do the same with the useEffect Hook.

Example Using Hooks

We’ve already seen this example at the top of this page, but let’s take a closer look at it:

import React, { useState, useEffect } from 'react';
function Example() {
  const [count, setCount] = useState(0);

  useEffect(() => {    document.title = `You clicked ${count} times`;  });
  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>
        Click me
      </button>
    </div>
  );
}

What does useEffect do? By using this Hook, you tell React that your component needs to do something after render. React will remember the function you passed (we’ll refer to it as our “effect”), and call it later after performing the DOM updates. In this effect, we set the document title, but we could also perform data fetching or call some other imperative API.

Why is useEffect called inside a component? Placing useEffect inside the component lets us access the count state variable (or any props) right from the effect. We don’t need a special API to read it — it’s already in the function scope. Hooks embrace JavaScript closures and avoid introducing React-specific APIs where JavaScript already provides a solution.

Does useEffect run after every render? Yes! By default, it runs both after the first render and after every update. (We will later talk about how to customize this.) Instead of thinking in terms of “mounting” and “updating”, you might find it easier to think that effects happen “after render”. React guarantees the DOM has been updated by the time it runs the effects.

Detailed Explanation

Now that we know more about effects, these lines should make sense:

function Example() {
  const [count, setCount] = useState(0);

  useEffect(() => {
    document.title = `You clicked ${count} times`;
  });
}

We declare the count state variable, and then we tell React we need to use an effect. We pass a function to the useEffect Hook. This function we pass is our effect. Inside our effect, we set the document title using the document.title browser API. We can read the latest count inside the effect because it’s in the scope of our function. When React renders our component, it will remember the effect we used, and then run our effect after updating the DOM. This happens for every render, including the first one.

Experienced JavaScript developers might notice that the function passed to useEffect is going to be different on every render. This is intentional. In fact, this is what lets us read the count value from inside the effect without worrying about it getting stale. Every time we re-render, we schedule a different effect, replacing the previous one. In a way, this makes the effects behave more like a part of the render result — each effect “belongs” to a particular render. We will see more clearly why this is useful later on this page.

Tip

Unlike componentDidMount or componentDidUpdate , effects scheduled with useEffect don’t block the browser from updating the screen. This makes your app feel more responsive. The majority of effects don’t need to happen synchronously. In the uncommon cases where they do (such as measuring the layout), there is a separate useLayoutEffect Hook with an API identical to useEffect .

Effects with Cleanup

Earlier, we looked at how to express side effects that don’t require any cleanup. However, some effects do. For example, we might want to set up a subscription to some external data source. In that case, it is important to clean up so that we don’t introduce a memory leak! Let’s compare how we can do it with classes and with Hooks.

Example Using Classes

In a React class, you would typically set up a subscription in componentDidMount , and clean it up in componentWillUnmount . For example, let’s say we have a ChatAPI module that lets us subscribe to a friend’s online status. Here’s how we might subscribe and display that status using a class:

class FriendStatus extends React.Component {
  constructor(props) {
    super(props);
    this.state = { isOnline: null };
    this.handleStatusChange = this.handleStatusChange.bind(this);
  }

  componentDidMount() {    ChatAPI.subscribeToFriendStatus(      this.props.friend.id,      this.handleStatusChange    );  }  componentWillUnmount() {    ChatAPI.unsubscribeFromFriendStatus(      this.props.friend.id,      this.handleStatusChange    );  }  handleStatusChange(status) {    this.setState({      isOnline: status.isOnline    });  }
  render() {
    if (this.state.isOnline === null) {
      return 'Loading...';
    }
    return this.state.isOnline ? 'Online' : 'Offline';
  }
}

Notice how componentDidMount and componentWillUnmount need to mirror each other. Lifecycle methods force us to split this logic even though conceptually code in both of them is related to the same effect.

Note

Eagle-eyed readers may notice that this example also needs a componentDidUpdate method to be fully correct. We’ll ignore this for now but will come back to it in a later section of this page.

Example Using Hooks

Let’s see how we could write this component with Hooks.

You might be thinking that we’d need a separate effect to perform the cleanup. But code for adding and removing a subscription is so tightly related that useEffect is designed to keep it together. If your effect returns a function, React will run it when it is time to clean up:

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

function FriendStatus(props) {
  const [isOnline, setIsOnline] = useState(null);

  useEffect(() => {    function handleStatusChange(status) {      setIsOnline(status.isOnline);    }    ChatAPI.subscribeToFriendStatus(props.friend.id, handleStatusChange);    // Specify how to clean up after this effect:    return function cleanup() {      ChatAPI.unsubscribeFromFriendStatus(props.friend.id, handleStatusChange);    };  });
  if (isOnline === null) {
    return 'Loading...';
  }
  return isOnline ? 'Online' : 'Offline';
}

Why did we return a function from our effect? This is the optional cleanup mechanism for effects. Every effect may return a function that cleans up after it. This lets us keep the logic for adding and removing subscriptions close to each other. They’re part of the same effect!

When exactly does React clean up an effect? React performs the cleanup when the component unmounts. However, as we learned earlier, effects run for every render and not just once. This is why React also cleans up effects from the previous render before running the effects next time. We’ll discuss why this helps avoid bugs and how to opt out of this behavior in case it creates performance issues later below.

Note

We don’t have to return a named function from the effect. We called it cleanup here to clarify its purpose, but you could return an arrow function or call it something different.

Recap

We’ve learned that useEffect lets us express different kinds of side effects after a component renders. Some effects might require cleanup so they return a function:

  useEffect(() => {
    function handleStatusChange(status) {
      setIsOnline(status.isOnline);
    }

    ChatAPI.subscribeToFriendStatus(props.friend.id, handleStatusChange);
    return () => {
      ChatAPI.unsubscribeFromFriendStatus(props.friend.id, handleStatusChange);
    };
  });

Other effects might not have a cleanup phase, and don’t return anything.

  useEffect(() => {
    document.title = `You clicked ${count} times`;
  });

The Effect Hook unifies both use cases with a single API.

If you feel like you have a decent grasp on how the Effect Hook works, or if you feel overwhelmed, you can jump to the next page about Rules of Hooks now.

Tips for Using Effects

We’ll continue this page with an in-depth look at some aspects of useEffect that experienced React users will likely be curious about. Don’t feel obligated to dig into them now. You can always come back to this page to learn more details about the Effect Hook.

Tip: Use Multiple Effects to Separate Concerns

One of the problems we outlined in the Motivation for Hooks is that class lifecycle methods often contain unrelated logic, but related logic gets broken up into several methods. Here is a component that combines the counter and the friend status indicator logic from the previous examples:

class FriendStatusWithCounter extends React.Component {
  constructor(props) {
    super(props);
    this.state = { count: 0, isOnline: null };
    this.handleStatusChange = this.handleStatusChange.bind(this);
  }

  componentDidMount() {
    document.title = `You clicked ${this.state.count} times`;
    ChatAPI.subscribeToFriendStatus(
      this.props.friend.id,
      this.handleStatusChange
    );
  }

  componentDidUpdate() {
    document.title = `You clicked ${this.state.count} times`;
  }

  componentWillUnmount() {
    ChatAPI.unsubscribeFromFriendStatus(
      this.props.friend.id,
      this.handleStatusChange
    );
  }

  handleStatusChange(status) {
    this.setState({
      isOnline: status.isOnline
    });
  }
  // ...

Note how the logic that sets document.title is split between componentDidMount and componentDidUpdate . The subscription logic is also spread between componentDidMount and componentWillUnmount . And componentDidMount contains code for both tasks.

So, how can Hooks solve this problem? Just like you can use the State Hook more than once, you can also use several effects. This lets us separate unrelated logic into different effects:

function FriendStatusWithCounter(props) {
  const [count, setCount] = useState(0);
  useEffect(() => {    document.title = `You clicked ${count} times`;
  });

  const [isOnline, setIsOnline] = useState(null);
  useEffect(() => {    function handleStatusChange(status) {
      setIsOnline(status.isOnline);
    }

    ChatAPI.subscribeToFriendStatus(props.friend.id, handleStatusChange);
    return () => {
      ChatAPI.unsubscribeFromFriendStatus(props.friend.id, handleStatusChange);
    };
  });
  // ...
}

Hooks let us split the code based on what it is doing rather than a lifecycle method name. React will apply every effect used by the component, in the order they were specified.

Explanation: Why Effects Run on Each Update

If you’re used to classes, you might be wondering why the effect cleanup phase happens after every re-render, and not just once during unmounting. Let’s look at a practical example to see why this design helps us create components with fewer bugs.

Earlier on this page, we introduced an example FriendStatus component that displays whether a friend is online or not. Our class reads friend.id from this.props , subscribes to the friend status after the component mounts, and unsubscribes during unmounting:

  componentDidMount() {
    ChatAPI.subscribeToFriendStatus(
      this.props.friend.id,
      this.handleStatusChange
    );
  }

  componentWillUnmount() {
    ChatAPI.unsubscribeFromFriendStatus(
      this.props.friend.id,
      this.handleStatusChange
    );
  }

But what happens if the friend prop changes while the component is on the screen? Our component would continue displaying the online status of a different friend. This is a bug. We would also cause a memory leak or crash when unmounting since the unsubscribe call would use the wrong friend ID.

In a class component, we would need to add componentDidUpdate to handle this case:

  componentDidMount() {
    ChatAPI.subscribeToFriendStatus(
      this.props.friend.id,
      this.handleStatusChange
    );
  }

  componentDidUpdate(prevProps) {    // Unsubscribe from the previous friend.id    ChatAPI.unsubscribeFromFriendStatus(      prevProps.friend.id,      this.handleStatusChange    );    // Subscribe to the next friend.id    ChatAPI.subscribeToFriendStatus(      this.props.friend.id,      this.handleStatusChange    );  }
  componentWillUnmount() {
    ChatAPI.unsubscribeFromFriendStatus(
      this.props.friend.id,
      this.handleStatusChange
    );
  }

Forgetting to handle componentDidUpdate properly is a common source of bugs in React applications.

Now consider the version of this component that uses Hooks:

function FriendStatus(props) {
  // ...
  useEffect(() => {
    // ...
    ChatAPI.subscribeToFriendStatus(props.friend.id, handleStatusChange);
    return () => {
      ChatAPI.unsubscribeFromFriendStatus(props.friend.id, handleStatusChange);
    };
  });

It doesn’t suffer from this bug. (But we also didn’t make any changes to it.)

There is no special code for handling updates because useEffect handles them by default . It cleans up the previous effects before applying the next effects. To illustrate this, here is a sequence of subscribe and unsubscribe calls that this component could produce over time:

// Mount with { friend: { id: 100 } } props
ChatAPI.subscribeToFriendStatus(100, handleStatusChange);     // Run first effect

// Update with { friend: { id: 200 } } props
ChatAPI.unsubscribeFromFriendStatus(100, handleStatusChange); // Clean up previous effect
ChatAPI.subscribeToFriendStatus(200, handleStatusChange);     // Run next effect

// Update with { friend: { id: 300 } } props
ChatAPI.unsubscribeFromFriendStatus(200, handleStatusChange); // Clean up previous effect
ChatAPI.subscribeToFriendStatus(300, handleStatusChange);     // Run next effect

// Unmount
ChatAPI.unsubscribeFromFriendStatus(300, handleStatusChange); // Clean up last effect

This behavior ensures consistency by default and prevents bugs that are common in class components due to missing update logic.

Tip: Optimizing Performance by Skipping Effects

In some cases, cleaning up or applying the effect after every render might create a performance problem. In class components, we can solve this by writing an extra comparison with prevProps or prevState inside componentDidUpdate :

componentDidUpdate(prevProps, prevState) {
  if (prevState.count !== this.state.count) {
    document.title = `You clicked ${this.state.count} times`;
  }
}

This requirement is common enough that it is built into the useEffect Hook API. You can tell React to skip applying an effect if certain values haven’t changed between re-renders. To do so, pass an array as an optional second argument to useEffect :

useEffect(() => {
  document.title = `You clicked ${count} times`;
}, [count]); // Only re-run the effect if count changes

In the example above, we pass [count] as the second argument. What does this mean? If the count is 5 , and then our component re-renders with count still equal to 5 , React will compare [5] from the previous render and [5] from the next render. Because all items in the array are the same ( 5 === 5 ), React would skip the effect. That’s our optimization.

When we render with count updated to 6 , React will compare the items in the [5] array from the previous render to items in the [6] array from the next render. This time, React will re-apply the effect because 5 !== 6 . If there are multiple items in the array, React will re-run the effect even if just one of them is different.

This also works for effects that have a cleanup phase:

useEffect(() => {
  function handleStatusChange(status) {
    setIsOnline(status.isOnline);
  }

  ChatAPI.subscribeToFriendStatus(props.friend.id, handleStatusChange);
  return () => {
    ChatAPI.unsubscribeFromFriendStatus(props.friend.id, handleStatusChange);
  };
}, [props.friend.id]); // Only re-subscribe if props.friend.id changes

In the future, the second argument might get added automatically by a build-time transformation.

Note

If you use this optimization, make sure the array includes all values from the component scope (such as props and state) that change over time and that are used by the effect . Otherwise, your code will reference stale values from previous renders. Learn more about how to deal with functions and what to do when the array changes too often.

If you want to run an effect and clean it up only once (on mount and unmount), you can pass an empty array ( [] ) as a second argument. This tells React that your effect doesn’t depend on any values from props or state, so it never needs to re-run. This isn’t handled as a special case — it follows directly from how the dependencies array always works.

If you pass an empty array ( [] ), the props and state inside the effect will always have their initial values. While passing [] as the second argument is closer to the familiar componentDidMount and componentWillUnmount mental model, there are usually better solutions to avoid re-running effects too often. Also, don’t forget that React defers running useEffect until after the browser has painted, so doing extra work is less of a problem.

We recommend using the exhaustive-deps rule as part of our eslint-plugin-react-hooks package. It warns when dependencies are specified incorrectly and suggests a fix.

Next Steps

Congratulations! This was a long page, but hopefully by the end most of your questions about effects were answered. You’ve learned both the State Hook and the Effect Hook, and there is a lot you can do with both of them combined. They cover most of the use cases for classes — and where they don’t, you might find the additional Hooks helpful.

We’re also starting to see how Hooks solve problems outlined in Motivation. We’ve seen how effect cleanup avoids duplication in componentDidUpdate and componentWillUnmount , brings related code closer together, and helps us avoid bugs. We’ve also seen how we can separate effects by their purpose, which is something we couldn’t do in classes at all.

At this point you might be questioning how Hooks work. How can React know which useState call corresponds to which state variable between re-renders? How does React “match up” previous and next effects on every update? On the next page we will learn about the Rules of Hooks — they’re essential to making Hooks work.

Hooks are a new addition in React 16.8. They let you use state and other React features without writing a class.

Hooks are JavaScript functions, but you need to follow two rules when using them. We provide a linter plugin to enforce these rules automatically:

Only Call Hooks at the Top Level

Don’t call Hooks inside loops, conditions, or nested functions. Instead, always use Hooks at the top level of your React function, before any early returns. By following this rule, you ensure that Hooks are called in the same order each time a component renders. That’s what allows React to correctly preserve the state of Hooks between multiple useState and useEffect calls. (If you’re curious, we’ll explain this in depth below.)

Only Call Hooks from React Functions

Don’t call Hooks from regular JavaScript functions. Instead, you can:

• ✅ Call Hooks from React function components.


• ✅ Call Hooks from custom Hooks (we’ll learn about them on the next page).

By following this rule, you ensure that all stateful logic in a component is clearly visible from its source code.

ESLint Plugin

We released an ESLint plugin called eslint-plugin-react-hooks that enforces these two rules. You can add this plugin to your project if you’d like to try it:

This plugin is included by default in Create React App.

npm install eslint-plugin-react-hooks --save-dev

// Your ESLint configuration
{
  "plugins": [
    // ...
    "react-hooks"
  ],
  "rules": {
    // ...
    "react-hooks/rules-of-hooks": "error", // Checks rules of Hooks
    "react-hooks/exhaustive-deps": "warn" // Checks effect dependencies
  }
}

You can skip to the next page explaining how to write your own Hooks now. On this page, we’ll continue by explaining the reasoning behind these rules.

Explanation

As we learned earlier, we can use multiple State or Effect Hooks in a single component:

function Form() {
  // 1. Use the name state variable
  const [name, setName] = useState('Mary');

  // 2. Use an effect for persisting the form
  useEffect(function persistForm() {
    localStorage.setItem('formData', name);
  });

  // 3. Use the surname state variable
  const [surname, setSurname] = useState('Poppins');

  // 4. Use an effect for updating the title
  useEffect(function updateTitle() {
    document.title = name + ' ' + surname;
  });

  // ...
}

So how does React know which state corresponds to which useState call? The answer is that React relies on the order in which Hooks are called . Our example works because the order of the Hook calls is the same on every render:

// ------------
// First render
// ------------
useState('Mary')           // 1. Initialize the name state variable with 'Mary'
useEffect(persistForm)     // 2. Add an effect for persisting the form
useState('Poppins')        // 3. Initialize the surname state variable with 'Poppins'
useEffect(updateTitle)     // 4. Add an effect for updating the title

// -------------
// Second render
// -------------
useState('Mary')           // 1. Read the name state variable (argument is ignored)
useEffect(persistForm)     // 2. Replace the effect for persisting the form
useState('Poppins')        // 3. Read the surname state variable (argument is ignored)
useEffect(updateTitle)     // 4. Replace the effect for updating the title

// ...

As long as the order of the Hook calls is the same between renders, React can associate some local state with each of them. But what happens if we put a Hook call (for example, the persistForm effect) inside a condition?

  // 🔴 We're breaking the first rule by using a Hook in a condition
  if (name !== '') {
    useEffect(function persistForm() {
      localStorage.setItem('formData', name);
    });
  }

The name !== '' condition is true on the first render, so we run this Hook. However, on the next render the user might clear the form, making the condition false . Now that we skip this Hook during rendering, the order of the Hook calls becomes different:

useState('Mary')           // 1. Read the name state variable (argument is ignored)
// useEffect(persistForm)  // 🔴 This Hook was skipped!
useState('Poppins')        // 🔴 2 (but was 3). Fail to read the surname state variable
useEffect(updateTitle)     // 🔴 3 (but was 4). Fail to replace the effect

React wouldn’t know what to return for the second useState Hook call. React expected that the second Hook call in this component corresponds to the persistForm effect, just like during the previous render, but it doesn’t anymore. From that point, every next Hook call after the one we skipped would also shift by one, leading to bugs.

This is why Hooks must be called on the top level of our components. If we want to run an effect conditionally, we can put that condition inside our Hook:

  useEffect(function persistForm() {
    // 👍 We're not breaking the first rule anymore
    if (name !== '') {
      localStorage.setItem('formData', name);
    }
  });

Note that you don’t need to worry about this problem if you use the provided lint rule. But now you also know why Hooks work this way, and which issues the rule is preventing.

Next Steps

Finally, we’re ready to learn about writing your own Hooks! Custom Hooks let you combine Hooks provided by React into your own abstractions, and reuse common stateful logic between different components.

Hooks are a new addition in React 16.8. They let you use state and other React features without writing a class.

Building your own Hooks lets you extract component logic into reusable functions.

When we were learning about using the Effect Hook, we saw this component from a chat application that displays a message indicating whether a friend is online or offline:

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

function FriendStatus(props) {
  const [isOnline, setIsOnline] = useState(null);  useEffect(() => {    function handleStatusChange(status) {      setIsOnline(status.isOnline);    }    ChatAPI.subscribeToFriendStatus(props.friend.id, handleStatusChange);    return () => {      ChatAPI.unsubscribeFromFriendStatus(props.friend.id, handleStatusChange);    };  });
  if (isOnline === null) {
    return 'Loading...';
  }
  return isOnline ? 'Online' : 'Offline';
}

Now let’s say that our chat application also has a contact list, and we want to render names of online users with a green color. We could copy and paste similar logic above into our FriendListItem component but it wouldn’t be ideal:

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

function FriendListItem(props) {
  const [isOnline, setIsOnline] = useState(null);  useEffect(() => {    function handleStatusChange(status) {      setIsOnline(status.isOnline);    }    ChatAPI.subscribeToFriendStatus(props.friend.id, handleStatusChange);    return () => {      ChatAPI.unsubscribeFromFriendStatus(props.friend.id, handleStatusChange);    };  });
  return (
    <li style={{ color: isOnline ? 'green' : 'black' }}>
      {props.friend.name}
    </li>
  );
}

Instead, we’d like to share this logic between FriendStatus and FriendListItem .

Traditionally in React, we’ve had two popular ways to share stateful logic between components: render props and higher-order components. We will now look at how Hooks solve many of the same problems without forcing you to add more components to the tree.

Extracting a Custom Hook

When we want to share logic between two JavaScript functions, we extract it to a third function. Both components and Hooks are functions, so this works for them too!

A custom Hook is a JavaScript function whose name starts with ” use ” and that may call other Hooks. For example, useFriendStatus below is our first custom Hook:

import { useState, useEffect } from 'react';

function useFriendStatus(friendID) {  const [isOnline, setIsOnline] = useState(null);

  useEffect(() => {
    function handleStatusChange(status) {
      setIsOnline(status.isOnline);
    }

    ChatAPI.subscribeToFriendStatus(friendID, handleStatusChange);
    return () => {
      ChatAPI.unsubscribeFromFriendStatus(friendID, handleStatusChange);
    };
  });

  return isOnline;
}

There’s nothing new inside of it — the logic is copied from the components above. Just like in a component, make sure to only call other Hooks unconditionally at the top level of your custom Hook.

Unlike a React component, a custom Hook doesn’t need to have a specific signature. We can decide what it takes as arguments, and what, if anything, it should return. In other words, it’s just like a normal function. Its name should always start with use so that you can tell at a glance that the rules of Hooks apply to it.

The purpose of our useFriendStatus Hook is to subscribe us to a friend’s status. This is why it takes friendID as an argument, and returns whether this friend is online:

function useFriendStatus(friendID) {
  const [isOnline, setIsOnline] = useState(null);

  // ...

  return isOnline;
}

Now let’s see how we can use our custom Hook.

Using a Custom Hook

In the beginning, our stated goal was to remove the duplicated logic from the FriendStatus and FriendListItem components. Both of them want to know whether a friend is online.

Now that we’ve extracted this logic to a useFriendStatus hook, we can just use it:

function FriendStatus(props) {
  const isOnline = useFriendStatus(props.friend.id);
  if (isOnline === null) {
    return 'Loading...';
  }
  return isOnline ? 'Online' : 'Offline';
}

function FriendListItem(props) {
  const isOnline = useFriendStatus(props.friend.id);
  return (
    <li style={{ color: isOnline ? 'green' : 'black' }}>
      {props.friend.name}
    </li>
  );
}

Is this code equivalent to the original examples? Yes, it works in exactly the same way. If you look closely, you’ll notice we didn’t make any changes to the behavior. All we did was to extract some common code between two functions into a separate function. Custom Hooks are a convention that naturally follows from the design of Hooks, rather than a React feature.

Do I have to name my custom Hooks starting with “ use ”? Please do. This convention is very important. Without it, we wouldn’t be able to automatically check for violations of rules of Hooks because we couldn’t tell if a certain function contains calls to Hooks inside of it.

Do two components using the same Hook share state? No. Custom Hooks are a mechanism to reuse stateful logic (such as setting up a subscription and remembering the current value), but every time you use a custom Hook, all state and effects inside of it are fully isolated.

How does a custom Hook get isolated state? Each call to a Hook gets isolated state. Because we call useFriendStatus directly, from React’s point of view our component just calls useState and useEffect . And as we learned earlier, we can call useState and useEffect many times in one component, and they will be completely independent.

Tip: Pass Information Between Hooks

Since Hooks are functions, we can pass information between them.

To illustrate this, we’ll use another component from our hypothetical chat example. This is a chat message recipient picker that displays whether the currently selected friend is online:

const friendList = [
  { id: 1, name: 'Phoebe' },
  { id: 2, name: 'Rachel' },
  { id: 3, name: 'Ross' },
];

function ChatRecipientPicker() {
  const [recipientID, setRecipientID] = useState(1);  const isRecipientOnline = useFriendStatus(recipientID);
  return (
    <>
      <Circle color={isRecipientOnline ? 'green' : 'red'} />      <select
        value={recipientID}
        onChange={e => setRecipientID(Number(e.target.value))}
      >
        {friendList.map(friend => (
          <option key={friend.id} value={friend.id}>
            {friend.name}
          </option>
        ))}
      </select>
    </>
  );
}

We keep the currently chosen friend ID in the recipientID state variable, and update it if the user chooses a different friend in the <select> picker.

Because the useState Hook call gives us the latest value of the recipientID state variable, we can pass it to our custom useFriendStatus Hook as an argument:

  const [recipientID, setRecipientID] = useState(1);
  const isRecipientOnline = useFriendStatus(recipientID);

This lets us know whether the currently selected friend is online. If we pick a different friend and update the recipientID state variable, our useFriendStatus Hook will unsubscribe from the previously selected friend, and subscribe to the status of the newly selected one.

useYourImagination()

Custom Hooks offer the flexibility of sharing logic that wasn’t possible in React components before. You can write custom Hooks that cover a wide range of use cases like form handling, animation, declarative subscriptions, timers, and probably many more we haven’t considered. What’s more, you can build Hooks that are just as easy to use as React’s built-in features.

Try to resist adding abstraction too early. Now that function components can do more, it’s likely that the average function component in your codebase will become longer. This is normal — don’t feel like you have to immediately split it into Hooks. But we also encourage you to start spotting cases where a custom Hook could hide complex logic behind a simple interface, or help untangle a messy component.

For example, maybe you have a complex component that contains a lot of local state that is managed in an ad-hoc way. useState doesn’t make centralizing the update logic any easier so you might prefer to write it as a Redux reducer:

function todosReducer(state, action) {
  switch (action.type) {
    case 'add':
      return [...state, {
        text: action.text,
        completed: false
      }];
    // ... other actions ...
    default:
      return state;
  }
}

Reducers are very convenient to test in isolation, and scale to express complex update logic. You can further break them apart into smaller reducers if necessary. However, you might also enjoy the benefits of using React local state, or might not want to install another library.

So what if we could write a useReducer Hook that lets us manage the local state of our component with a reducer? A simplified version of it might look like this:

function useReducer(reducer, initialState) {
  const [state, setState] = useState(initialState);

  function dispatch(action) {
    const nextState = reducer(state, action);
    setState(nextState);
  }

  return [state, dispatch];
}

Now we could use it in our component, and let the reducer drive its state management:

function Todos() {
  const [todos, dispatch] = useReducer(todosReducer, []);
  function handleAddClick(text) {
    dispatch({ type: 'add', text });
  }

  // ...
}

The need to manage local state with a reducer in a complex component is common enough that we’ve built the useReducer Hook right into React. You’ll find it together with other built-in Hooks in the Hooks API reference.

Hooks are a new addition in React 16.8. They let you use state and other React features without writing a class.

This page describes the APIs for the built-in Hooks in React.

If you’re new to Hooks, you might want to check out the overview first. You may also find useful information in the frequently asked questions section.

• 
  

  Basic Hooks

  
  
  
  
  • useState
  
  
  • useEffect
  
  
  • useContext
  
  
  
  


• 
  

  Additional Hooks

  
  
  
  
  • useReducer
  
  
  • useCallback
  
  
  • useMemo
  
  
  • useRef
  
  
  • useImperativeHandle
  
  
  • useLayoutEffect
  
  
  • useDebugValue
  
  
  • useDeferredValue
  
  
  • useTransition
  
  
  • useId
  
  
  
  


• 
  

  Library Hooks

  
  
  
  
  • useSyncExternalStore
  
  
  • useInsertionEffect
  
  
  
  

Basic Hooks

useState

const [state, setState] = useState(initialState);

Returns a stateful value, and a function to update it.

During the initial render, the returned state ( state ) is the same as the value passed as the first argument ( initialState ).

The setState function is used to update the state. It accepts a new state value and enqueues a re-render of the component.

setState(newState);

During subsequent re-renders, the first value returned by useState will always be the most recent state after applying updates.

Note

React guarantees that setState function identity is stable and won’t change on re-renders. This is why it’s safe to omit from the useEffect or useCallback dependency list.

Functional updates

If the new state is computed using the previous state, you can pass a function to setState . The function will receive the previous value, and return an updated value. Here’s an example of a counter component that uses both forms of setState :

function Counter({initialCount}) {
  const [count, setCount] = useState(initialCount);
  return (
    <>
      Count: {count}
      <button onClick={() => setCount(initialCount)}>Reset</button>
      <button onClick={() => setCount(prevCount => prevCount - 1)}>-</button>
      <button onClick={() => setCount(prevCount => prevCount + 1)}>+</button>
    </>
  );
}

The ”+” and ”-” buttons use the functional form, because the updated value is based on the previous value. But the “Reset” button uses the normal form, because it always sets the count back to the initial value.

If your update function returns the exact same value as the current state, the subsequent rerender will be skipped completely.

Note

Unlike the setState method found in class components, useState does not automatically merge update objects. You can replicate this behavior by combining the function updater form with object spread syntax:

const [state, setState] = useState({});
setState(prevState => {
  // Object.assign would also work
  return {...prevState, ...updatedValues};
});

Another option is useReducer , which is more suited for managing state objects that contain multiple sub-values.

Lazy initial state

The initialState argument is the state used during the initial render. In subsequent renders, it is disregarded. If the initial state is the result of an expensive computation, you may provide a function instead, which will be executed only on the initial render:

const [state, setState] = useState(() => {
  const initialState = someExpensiveComputation(props);
  return initialState;
});

Bailing out of a state update

If you update a State Hook to the same value as the current state, React will bail out without rendering the children or firing effects. (React uses the Object.is comparison algorithm.)

Note that React may still need to render that specific component again before bailing out. That shouldn’t be a concern because React won’t unnecessarily go “deeper” into the tree. If you’re doing expensive calculations while rendering, you can optimize them with useMemo .

Batching of state updates

React may group several state updates into a single re-render to improve performance. Normally, this improves performance and shouldn’t affect your application’s behavior.

Before React 18, only updates inside React event handlers were batched. Starting with React 18, batching is enabled for all updates by default. Note that React makes sure that updates from several different user-initiated events — for example, clicking a button twice — are always processed separately and do not get batched. This prevents logical mistakes.

In the rare case that you need to force the DOM update to be applied synchronously, you may wrap it in flushSync . However, this can hurt performance so do this only where needed.

useEffect

useEffect(didUpdate);

Accepts a function that contains imperative, possibly effectful code.

Mutations, subscriptions, timers, logging, and other side effects are not allowed inside the main body of a function component (referred to as React’s render phase ). Doing so will lead to confusing bugs and inconsistencies in the UI.

Instead, use useEffect . The function passed to useEffect will run after the render is committed to the screen. Think of effects as an escape hatch from React’s purely functional world into the imperative world.

By default, effects run after every completed render, but you can choose to fire them only when certain values have changed.

Cleaning up an effect

Often, effects create resources that need to be cleaned up before the component leaves the screen, such as a subscription or timer ID. To do this, the function passed to useEffect may return a clean-up function. For example, to create a subscription:

useEffect(() => {
  const subscription = props.source.subscribe();
  return () => {
    // Clean up the subscription
    subscription.unsubscribe();
  };
});

The clean-up function runs before the component is removed from the UI to prevent memory leaks. Additionally, if a component renders multiple times (as they typically do), the previous effect is cleaned up before executing the next effect . In our example, this means a new subscription is created on every update. To avoid firing an effect on every update, refer to the next section.

Timing of effects

Unlike componentDidMount and componentDidUpdate , the function passed to useEffect fires after layout and paint, during a deferred event. This makes it suitable for the many common side effects, like setting up subscriptions and event handlers, because most types of work shouldn’t block the browser from updating the screen.

However, not all effects can be deferred. For example, a DOM mutation that is visible to the user must fire synchronously before the next paint so that the user does not perceive a visual inconsistency. (The distinction is conceptually similar to passive versus active event listeners.) For these types of effects, React provides one additional Hook called useLayoutEffect . It has the same signature as useEffect , and only differs in when it is fired.

Additionally, starting in React 18, the function passed to useEffect will fire synchronously before layout and paint when it’s the result of a discrete user input such as a click, or when it’s the result of an update wrapped in flushSync . This behavior allows the result of the effect to be observed by the event system, or by the caller of flushSync .

Note

This only affects the timing of when the function passed to useEffect is called - updates scheduled inside these effects are still deferred. This is different than useLayoutEffect , which fires the function and processes the updates inside of it immediately.

Even in cases where useEffect is deferred until after the browser has painted, it’s guaranteed to fire before any new renders. React will always flush a previous render’s effects before starting a new update.

Conditionally firing an effect

The default behavior for effects is to fire the effect after every completed render. That way an effect is always recreated if one of its dependencies changes.

However, this may be overkill in some cases, like the subscription example from the previous section. We don’t need to create a new subscription on every update, only if the source prop has changed.

To implement this, pass a second argument to useEffect that is the array of values that the effect depends on. Our updated example now looks like this:

useEffect(
  () => {
    const subscription = props.source.subscribe();
    return () => {
      subscription.unsubscribe();
    };
  },
  [props.source],
);

Now the subscription will only be recreated when props.source changes.

Note

If you use this optimization, make sure the array includes all values from the component scope (such as props and state) that change over time and that are used by the effect . Otherwise, your code will reference stale values from previous renders. Learn more about how to deal with functions and what to do when the array values change too often.

If you want to run an effect and clean it up only once (on mount and unmount), you can pass an empty array ( [] ) as a second argument. This tells React that your effect doesn’t depend on any values from props or state, so it never needs to re-run. This isn’t handled as a special case — it follows directly from how the dependencies array always works.

If you pass an empty array ( [] ), the props and state inside the effect will always have their initial values. While passing [] as the second argument is closer to the familiar componentDidMount and componentWillUnmount mental model, there are usually better solutions to avoid re-running effects too often. Also, don’t forget that React defers running useEffect until after the browser has painted, so doing extra work is less of a problem.

We recommend using the exhaustive-deps rule as part of our eslint-plugin-react-hooks package. It warns when dependencies are specified incorrectly and suggests a fix.

The array of dependencies is not passed as arguments to the effect function. Conceptually, though, that’s what they represent: every value referenced inside the effect function should also appear in the dependencies array. In the future, a sufficiently advanced compiler could create this array automatically.

useContext

const value = useContext(MyContext);

Accepts a context object (the value returned from React.createContext ) and returns the current context value for that context. The current context value is determined by the value prop of the nearest <MyContext.Provider> above the calling component in the tree.

When the nearest <MyContext.Provider> above the component updates, this Hook will trigger a rerender with the latest context value passed to that MyContext provider. Even if an ancestor uses React.memo or shouldComponentUpdate , a rerender will still happen starting at the component itself using useContext .

Don’t forget that the argument to useContext must be the context object itself :

• Correct: useContext(MyContext)


• Incorrect: useContext(MyContext.Consumer)


• Incorrect: useContext(MyContext.Provider)

A component calling useContext will always re-render when the context value changes. If re-rendering the component is expensive, you can optimize it by using memoization.

Tip

If you’re familiar with the context API before Hooks, useContext(MyContext) is equivalent to static contextType = MyContext in a class, or to <MyContext.Consumer> .

useContext(MyContext) only lets you read the context and subscribe to its changes. You still need a <MyContext.Provider> above in the tree to provide the value for this context.

Putting it together with Context.Provider

const themes = {
  light: {
    foreground: "#000000",
    background: "#eeeeee"
  },
  dark: {
    foreground: "#ffffff",
    background: "#222222"
  }
};

const ThemeContext = React.createContext(themes.light);

function App() {
  return (
    <ThemeContext.Provider value={themes.dark}>
      <Toolbar />
    </ThemeContext.Provider>
  );
}

function Toolbar(props) {
  return (
    <div>
      <ThemedButton />
    </div>
  );
}

function ThemedButton() {
  const theme = useContext(ThemeContext);  return (    <button style={{ background: theme.background, color: theme.foreground }}>      I am styled by theme context!    </button>  );
}

This example is modified for hooks from a previous example in the Context Advanced Guide, where you can find more information about when and how to use Context.

Additional Hooks

The following Hooks are either variants of the basic ones from the previous section, or only needed for specific edge cases. Don’t stress about learning them up front.

useReducer

const [state, dispatch] = useReducer(reducer, initialArg, init);

An alternative to useState . Accepts a reducer of type (state, action) => newState , and returns the current state paired with a dispatch method. (If you’re familiar with Redux, you already know how this works.)

useReducer is usually preferable to useState when you have complex state logic that involves multiple sub-values or when the next state depends on the previous one. useReducer also lets you optimize performance for components that trigger deep updates because you can pass dispatch down instead of callbacks.

Here’s the counter example from the useState section, rewritten to use a reducer:

const initialState = {count: 0};

function reducer(state, action) {
  switch (action.type) {
    case 'increment':
      return {count: state.count + 1};
    case 'decrement':
      return {count: state.count - 1};
    default:
      throw new Error();
  }
}

function Counter() {
  const [state, dispatch] = useReducer(reducer, initialState);
  return (
    <>
      Count: {state.count}
      <button onClick={() => dispatch({type: 'decrement'})}>-</button>
      <button onClick={() => dispatch({type: 'increment'})}>+</button>
    </>
  );
}

Note

React guarantees that dispatch function identity is stable and won’t change on re-renders. This is why it’s safe to omit from the useEffect or useCallback dependency list.

Specifying the initial state

There are two different ways to initialize useReducer state. You may choose either one depending on the use case. The simplest way is to pass the initial state as a second argument:

  const [state, dispatch] = useReducer(
    reducer,
    {count: initialCount}  );

Note

React doesn’t use the state = initialState argument convention popularized by Redux. The initial value sometimes needs to depend on props and so is specified from the Hook call instead. If you feel strongly about this, you can call useReducer(reducer, undefined, reducer) to emulate the Redux behavior, but it’s not encouraged.

Lazy initialization

You can also create the initial state lazily. To do this, you can pass an init function as the third argument. The initial state will be set to init(initialArg) .

It lets you extract the logic for calculating the initial state outside the reducer. This is also handy for resetting the state later in response to an action:

function init(initialCount) {  return {count: initialCount};}
function reducer(state, action) {
  switch (action.type) {
    case 'increment':
      return {count: state.count + 1};
    case 'decrement':
      return {count: state.count - 1};
    case 'reset':      return init(action.payload);    default:
      throw new Error();
  }
}

function Counter({initialCount}) {
  const [state, dispatch] = useReducer(reducer, initialCount, init);  return (
    <>
      Count: {state.count}
      <button
        onClick={() => dispatch({type: 'reset', payload: initialCount})}>        Reset
      </button>
      <button onClick={() => dispatch({type: 'decrement'})}>-</button>
      <button onClick={() => dispatch({type: 'increment'})}>+</button>
    </>
  );
}

Bailing out of a dispatch

If you return the same value from a Reducer Hook as the current state, React will bail out without rendering the children or firing effects. (React uses the Object.is comparison algorithm.)

Note that React may still need to render that specific component again before bailing out. That shouldn’t be a concern because React won’t unnecessarily go “deeper” into the tree. If you’re doing expensive calculations while rendering, you can optimize them with useMemo .

useCallback

const memoizedCallback = useCallback(
  () => {
    doSomething(a, b);
  },
  [a, b],
);

Returns a memoized callback.

Pass an inline callback and an array of dependencies. useCallback will return a memoized version of the callback that only changes if one of the dependencies has changed. This is useful when passing callbacks to optimized child components that rely on reference equality to prevent unnecessary renders (e.g. shouldComponentUpdate ).

useCallback(fn, deps) is equivalent to useMemo(() => fn, deps) .

Note

The array of dependencies is not passed as arguments to the callback. Conceptually, though, that’s what they represent: every value referenced inside the callback should also appear in the dependencies array. In the future, a sufficiently advanced compiler could create this array automatically.

We recommend using the exhaustive-deps rule as part of our eslint-plugin-react-hooks package. It warns when dependencies are specified incorrectly and suggests a fix.

useMemo

const memoizedValue = useMemo(() => computeExpensiveValue(a, b), [a, b]);

Returns a memoized value.

Pass a “create” function and an array of dependencies. useMemo will only recompute the memoized value when one of the dependencies has changed. This optimization helps to avoid expensive calculations on every render.

Remember that the function passed to useMemo runs during rendering. Don’t do anything there that you wouldn’t normally do while rendering. For example, side effects belong in useEffect , not useMemo .

If no array is provided, a new value will be computed on every render.

You may rely on useMemo as a performance optimization, not as a semantic guarantee. In the future, React may choose to “forget” some previously memoized values and recalculate them on next render, e.g. to free memory for offscreen components. Write your code so that it still works without useMemo — and then add it to optimize performance.

Note

The array of dependencies is not passed as arguments to the function. Conceptually, though, that’s what they represent: every value referenced inside the function should also appear in the dependencies array. In the future, a sufficiently advanced compiler could create this array automatically.

We recommend using the exhaustive-deps rule as part of our eslint-plugin-react-hooks package. It warns when dependencies are specified incorrectly and suggests a fix.

useRef

const refContainer = useRef(initialValue);

useRef returns a mutable ref object whose .current property is initialized to the passed argument ( initialValue ). The returned object will persist for the full lifetime of the component.

A common use case is to access a child imperatively:

function TextInputWithFocusButton() {
  const inputEl = useRef(null);
  const onButtonClick = () => {
    // `current` points to the mounted text input element
    inputEl.current.focus();
  };
  return (
    <>
      <input ref={inputEl} type="text" />
      <button onClick={onButtonClick}>Focus the input</button>
    </>
  );
}

Essentially, useRef is like a “box” that can hold a mutable value in its .current property.

You might be familiar with refs primarily as a way to access the DOM. If you pass a ref object to React with <div ref={myRef} /> , React will set its .current property to the corresponding DOM node whenever that node changes.

However, useRef() is useful for more than the ref attribute. It’s handy for keeping any mutable value around similar to how you’d use instance fields in classes.

This works because useRef() creates a plain JavaScript object. The only difference between useRef() and creating a {current: ...} object yourself is that useRef will give you the same ref object on every render.

Keep in mind that useRef doesn’t notify you when its content changes. Mutating the .current property doesn’t cause a re-render. If you want to run some code when React attaches or detaches a ref to a DOM node, you may want to use a callback ref instead.

useImperativeHandle

useImperativeHandle(ref, createHandle, [deps])

useImperativeHandle customizes the instance value that is exposed to parent components when using ref . As always, imperative code using refs should be avoided in most cases. useImperativeHandle should be used with forwardRef :

function FancyInput(props, ref) {
  const inputRef = useRef();
  useImperativeHandle(ref, () => ({
    focus: () => {
      inputRef.current.focus();
    }
  }));
  return <input ref={inputRef} ... />;
}
FancyInput = forwardRef(FancyInput);

In this example, a parent component that renders <FancyInput ref={inputRef} /> would be able to call inputRef.current.focus() .

useLayoutEffect

The signature is identical to useEffect , but it fires synchronously after all DOM mutations. Use this to read layout from the DOM and synchronously re-render. Updates scheduled inside useLayoutEffect will be flushed synchronously, before the browser has a chance to paint.

Prefer the standard useEffect when possible to avoid blocking visual updates.

Tip

If you’re migrating code from a class component, note useLayoutEffect fires in the same phase as componentDidMount and componentDidUpdate . However, we recommend starting with useEffect first and only trying useLayoutEffect if that causes a problem.

If you use server rendering, keep in mind that neither useLayoutEffect nor useEffect can run until the JavaScript is downloaded. This is why React warns when a server-rendered component contains useLayoutEffect . To fix this, either move that logic to useEffect (if it isn’t necessary for the first render), or delay showing that component until after the client renders (if the HTML looks broken until useLayoutEffect runs).

To exclude a component that needs layout effects from the server-rendered HTML, render it conditionally with showChild && <Child /> and defer showing it with useEffect(() => { setShowChild(true); }, []) . This way, the UI doesn’t appear broken before hydration.

useDebugValue

useDebugValue(value)

useDebugValue can be used to display a label for custom hooks in React DevTools.

For example, consider the useFriendStatus custom Hook described in “Building Your Own Hooks”:

function useFriendStatus(friendID) {
  const [isOnline, setIsOnline] = useState(null);

  // ...

  // Show a label in DevTools next to this Hook  // e.g. "FriendStatus: Online"  useDebugValue(isOnline ? 'Online' : 'Offline');
  return isOnline;
}

Tip

We don’t recommend adding debug values to every custom Hook. It’s most valuable for custom Hooks that are part of shared libraries.

Defer formatting debug values

In some cases formatting a value for display might be an expensive operation. It’s also unnecessary unless a Hook is actually inspected.

For this reason useDebugValue accepts a formatting function as an optional second parameter. This function is only called if the Hooks are inspected. It receives the debug value as a parameter and should return a formatted display value.

For example a custom Hook that returned a Date value could avoid calling the toDateString function unnecessarily by passing the following formatter:

useDebugValue(date, date => date.toDateString());

useDeferredValue

const deferredValue = useDeferredValue(value);

useDeferredValue accepts a value and returns a new copy of the value that will defer to more urgent updates. If the current render is the result of an urgent update, like user input, React will return the previous value and then render the new value after the urgent render has completed.

This hook is similar to user-space hooks which use debouncing or throttling to defer updates. The benefits to using useDeferredValue is that React will work on the update as soon as other work finishes (instead of waiting for an arbitrary amount of time), and like startTransition , deferred values can suspend without triggering an unexpected fallback for existing content.

Memoizing deferred children

useDeferredValue only defers the value that you pass to it. If you want to prevent a child component from re-rendering during an urgent update, you must also memoize that component with React.memo or React.useMemo :

function Typeahead() {
  const query = useSearchQuery('');
  const deferredQuery = useDeferredValue(query);

  // Memoizing tells React to only re-render when deferredQuery changes,
  // not when query changes.
  const suggestions = useMemo(() =>
    <SearchSuggestions query={deferredQuery} />,
    [deferredQuery]
  );

  return (
    <>
      <SearchInput query={query} />
      <Suspense fallback="Loading results...">
        {suggestions}
      </Suspense>
    </>
  );
}

Memoizing the children tells React that it only needs to re-render them when deferredQuery changes and not when query changes. This caveat is not unique to useDeferredValue , and it’s the same pattern you would use with similar hooks that use debouncing or throttling.

useTransition

const [isPending, startTransition] = useTransition();

Returns a stateful value for the pending state of the transition, and a function to start it.

startTransition lets you mark updates in the provided callback as transitions:

startTransition(() => {
  setCount(count + 1);
});

isPending indicates when a transition is active to show a pending state:

function App() {
  const [isPending, startTransition] = useTransition();
  const [count, setCount] = useState(0);
  
  function handleClick() {
    startTransition(() => {
      setCount(c => c + 1);
    });
  }

  return (
    <div>
      {isPending && <Spinner />}
      <button onClick={handleClick}>{count}</button>
    </div>
  );
}

Note:

Updates in a transition yield to more urgent updates such as clicks.

Updates in a transition will not show a fallback for re-suspended content. This allows the user to continue interacting with the current content while rendering the update.

useId

const id = useId();

useId is a hook for generating unique IDs that are stable across the server and client, while avoiding hydration mismatches.

Note

useId is not for generating keys in a list. Keys should be generated from your data.

For a basic example, pass the id directly to the elements that need it:

function Checkbox() {
  const id = useId();
  return (
    <>
      <label htmlFor={id}>Do you like React?</label>
      <input id={id} type="checkbox" name="react"/>
    </>
  );
};

For multiple IDs in the same component, append a suffix using the same id :

function NameFields() {
  const id = useId();
  return (
    <div>
      <label htmlFor={id + '-firstName'}>First Name</label>
      <div>
        <input id={id + '-firstName'} type="text" />
      </div>
      <label htmlFor={id + '-lastName'}>Last Name</label>
      <div>
        <input id={id + '-lastName'} type="text" />
      </div>
    </div>
  );
}