javascript - Strange behavior of React hooks: delayed data update

Question

Strange behavior: I expect that the first and the second console.log display a different result, but they display the same result and the value is changed only on the next click. How should I edit my code so that the value will be different?

function App() {
  const [count, setCount] = React.useState(false);
  const test = () => {
    console.log(count); //false
    setCount(!count);
    console.log(count); //false
  };
  return (
    <div className="App">
      <h1 onClick={test}>Hello StackOverflow</h1>
    </div>
  );
}

const rootElement = document.getElementById("root");
ReactDOM.render(<App />, rootElement);

<script crossorigin src="https://unpkg.com/react@16/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom@16/umd/react-dom.development.js"></script>
<div id="root"></div>

Answer 1

The state update is asynchronous so if you are using Hooks you can use useEffect to be notified after an update.

Here is an example:

https://codesandbox.io/s/wxy342m6l

If you are using setState of a React component, you can use the callback

this.setState((state) => ({count: !state.count}), () => console.log('Updated', this.state.count));

Remember to use the callback to update state if you need a state value.