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

Theintroduction page used this example to get familiar with Hooks:

import React,{ useState}from'react';functionExample(){// Declare a new state variable, which we'll call "count"const[count, setCount]=useState(0);return(<div><p>You clicked{count} times</p><buttononClick={()=>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:

classExampleextendsReact.Component{constructor(props){super(props);this.state={count:0};}render(){return(<div><p>You clicked{this.state.count} times</p><buttononClick={()=>this.setState({count:this.state.count+1})}>          Click me</button></div>);}}

The state starts as{ count: 0 }, and we incrementstate.count when the user clicks a button by callingthis.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:

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

or this:

functionExample(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”.

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

What’s a Hook?

Our new example starts by importing theuseState Hook from React:

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

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 inRules of Hooks.

Declaring a State Variable

In a class, we initialize thecount state to0 by settingthis.state to{ count: 0 } in the constructor:

classExampleextendsReact.Component{constructor(props){super(props);this.state={count:0};}

In a function component, we have nothis, so we can’t assign or readthis.state. Instead, we call theuseState Hook directly inside our component:

import React,{ useState}from'react';functionExample(){// Declare a new state variable, which we'll call "count"const[count, setCount]=useState(0);

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

What do we pass touseState as an argument? The only argument to theuseState() 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 pass0 as initial state for our variable. (If we wanted to store two different values in state, we would calluseState() twice.)

What doesuseState return? It returns a pair of values: the current state and a function that updates it. This is why we writeconst [count, setCount] = useState(). This is similar tothis.state.count andthis.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 itat the bottom of this page.

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

import React,{ useState}from'react';functionExample(){// Declare a new state variable, which we'll call "count"const[count, setCount]=useState(0);

We declare a state variable calledcount, and set it to0. React will remember its current value between re-renders, and provide the most recent one to our function. If we want to update the currentcount, we can callsetCount.

Note

You might be wondering: why isuseState not namedcreateState 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 namesalways start withuse. We’ll learn why later in theRules of Hooks.

Reading State

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

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

In a function, we can usecount directly:

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

Updating State

In a class, we need to callthis.setState() to update thecount state:

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

In a function, we already havesetCount andcount as variables so we don’t needthis:

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

Recap

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

1:import React,{ useState}from'react';2:3:functionExample(){4:const[count, setCount]=useState(0);5:6:return(7:<div> 8:<p>You clicked{count} times</p> 9:<buttononClick={()=>setCount(count+1)}>10:         Click me11:</button>12:</div>13:);14:}

• Line 1: We import theuseState Hook from React. It lets us keep local state in a function component.
• Line 4: Inside theExample component, we declare a new state variable by calling theuseState Hook. It returns a pair of values, to which we give names. We’re calling our variablecount because it holds the number of button clicks. We initialize it to zero by passing0 as the onlyuseState argument. The second returned item is itself a function. It lets us update thecount so we’ll name itsetCount.
• Line 9: When the user clicks, we callsetCount with a new value. React will then re-render theExample component, passing the newcount 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 variablesfruit andsetFruit, wherefruit is set to the first value returned byuseState, andsetFruit is the second. It is equivalent to this code:

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

When we declare a state variable withuseState, 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 componentuseState corresponds to since we’re not passing anything likethis back to React. We’ll answerthis 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 givedifferent names to different state variables if we want to use more than one:

functionExampleWithManyStates(){// 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 haveage,fruit, andtodos as local variables, and we can update them individually:

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

Youdon’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, unlikethis.setState in a class, updating a state variable alwaysreplaces it instead of merging it.

We provide more recommendations on splitting independent state variablesin the FAQ.

Next Steps

On this page we’ve learned about one of the Hooks provided by React, calleduseState. 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 withuse, and there are more Hooks we haven’t seen yet.

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