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

import React, { useState } from 'react';
function Example() {
  // ...
}
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);
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);
    };
  });
  // ...
}
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>
  );
}
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>
    );
  }
}
As a reminder, function components in React look like this:

const Example = (props) => {
  // You can use Hooks here!
  return <div />;
}
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    };  }
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' }]);
When we want to display the current count in a class, we read this.state.count:

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

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