Hooks at a Glance
Tags: react, javascript
Published: 2021-12-22 11:11:00

Hooks are backwards-compatible. This page provides an overview of Hooks for experienced React users. This is a fast-paced overview.

State Hook:
This example renders a counter. When you click the button, it increments the value:

```
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>
  );
}

```
Here, useState is a Hook (we’ll talk about what this means in a moment). We call it inside a function component to add some local state to it. React will preserve this state between re-renders. useState returns a pair: the current state value and a function that lets you update it. You can call this function from an event handler or somewhere else. It’s similar to this.setState in a class, except it doesn’t merge the old and new state together. (We’ll show an example comparing useState to this.state in Using the State Hook.)

The only argument to useState is the initial state. In the example above, it is 0 because our counter starts from zero. Note that unlike this.state, the state here doesn’t have to be an object — although it can be if you want. The initial state argument is only used during the first render.

Declaring multiple state variables
You can use the State Hook more than once in a single component:

```
function ExampleWithManyStates() {
  // Declare multiple state variables!
  const [age, setAge] = useState(42);
  const [fruit, setFruit] = useState('banana');
  const [todos, setTodos] = useState([{ text: 'Learn Hooks' }]);
  // ...
}
```
The array destructuring syntax lets us give different names to the state variables we declared by calling useState. These names aren’t a part of the useState API. Instead, React assumes that if you call useState many times, you do it in the same order during every render. We’ll come back to why this works and when this is useful later.

But what is a Hook?
Hooks are functions that let you “hook into” React state and lifecycle features from function components. Hooks don’t work inside classes — they let you use React without classes. (We don’t recommend rewriting your existing components overnight but you can start using Hooks in the new ones if you’d like.)
React provides a few built-in Hooks like useState. You can also create your own Hooks to reuse stateful behavior between different components. We’ll look at the built-in Hooks first.

Effect Hook
You’ve likely performed data fetching, subscriptions, or manually changing the DOM from React components before. We call these operations “side effects” (or “effects” for short) because they can affect other components and can’t be done during rendering.
he Effect Hook, useEffect, adds the ability to perform side effects from a function component. It serves the same purpose as componentDidMount, componentDidUpdate, and componentWillUnmount in React classes, but unified into a single API. (We’ll show examples comparing useEffect to these methods in Using the Effect Hook.)
For example, this component sets the document title after React updates the DOM:

```
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>
  );
}
```
When you call useEffect, you’re telling React to run your “effect” function after flushing changes to the DOM. Effects are declared inside the component so they have access to its props and state. By default, React runs the effects after every render — including the first render. (We’ll talk more about how this compares to class lifecycles in Using the Effect Hook.)
Effects may also optionally specify how to “clean up” after them by returning a function. For example, this component uses an effect to subscribe to a friend’s online status, and cleans up by unsubscribing from it:

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

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

  function handleStatusChange(status) {
    setIsOnline(status.isOnline);
  }

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

  if (isOnline === null) {
    return 'Loading...';
  }
  return isOnline ? 'Online' : 'Offline';
}
```
In this example, React would unsubscribe from our ChatAPI when the component unmounts, as well as before re-running the effect due to a subsequent render. (If you want, there’s a way to tell React to skip re-subscribing if the props.friend.id we passed to ChatAPI didn’t change.)
Just like with useState, you can use more than a single effect in a component:

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

  const [isOnline, setIsOnline] = useState(null);
  useEffect(() => {
    ChatAPI.subscribeToFriendStatus(props.friend.id, handleStatusChange);
    return () => {
      ChatAPI.unsubscribeFromFriendStatus(props.friend.id, handleStatusChange);
    };
  });

  function handleStatusChange(status) {
    setIsOnline(status.isOnline);
  }
```
Hooks let you organize side effects in a component by what pieces are related (such as adding and removing a subscription), rather than forcing a split based on lifecycle methods.

Rules of Hooks
Hooks are JavaScript functions, but they impose two additional rules:
Only call Hooks at the top level. Don’t call Hooks inside loops, conditions, or nested functions.
Only call Hooks from React function components. Don’t call Hooks from regular JavaScript functions. (There is just one other valid place to call Hooks — your own custom Hooks. We’ll learn about them in a moment.)
We provide a linter plugin to enforce these rules automatically. We understand these rules might seem limiting or confusing at first, but they are essential to making Hooks work well.