React - useState - why setTimeout function does not have latest state value?

Question

Recently I was working on React Hooks and got stuck with one problem/doubt?

Below is a basic implementation to reproduce the issue, Here I'm just toggling flag (a state) variable on click of the button.

  const [flag, toggleFlag] = useState(false);   const data = useRef(null);   data.current = flag;    const _onClick = () => {     toggleFlag(!flag);     // toggleFlag(!data.current); // working      setTimeout(() => {       toggleFlag(!flag); // does not have latest value, why ?       // toggleFlag(!data.current); // working     }, 2000);   };    return (     <div className="App">       <button onClick={_onClick}>{flag ? "true" : "false"}</button>     </div>   ); 

I figured out some other way to overcome this problem like using useRef or useReducer, but is this correct or is there any other way to solve this with useState only?

Also, it would be really helpful if anyone explains why we get old value of state inside the setTimeout.

Sandbox URL - https://codesandbox.io/s/xp540ynomo

Answer 1

This boils down to how closures work in JavaScript. The function given to setTimeout will get the flag variable from the initial render, since flag is not mutated.

You could instead give a function as argument to toggleFlag. This function will get the correct flag value as argument, and what is returned from this function is what will replace the state.

Example

const { useState } = React;    function App() {    const [flag, toggleFlag] = useState(false);      const _onClick = () => {      toggleFlag(!flag);        setTimeout(() => {        toggleFlag(flag => !flag)      }, 2000);    };      return (      <div className="App">        <button onClick={_onClick}>{flag ? "true" : "false"}</button>      </div>    );  }    ReactDOM.render(<App />, document.getElementById("root"));

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

Answer 2

The function given to setTimeout will get the flag variable from the _onClick function. The _onClick function gets created every render and "stores" the value which the flag variable gets on this render.

function App() {   const [flag, toggleFlag] = useState(false);   console.log("App thinks that flag is", flag);    const _onClick = () => {     console.log("_onClick thinks that flag is", flag);     toggleFlag(!flag);      setTimeout(() => {       console.log("setTimeout thinks that flag is", flag);     }, 100);   };    return (     <div className="App">       <button onClick={_onClick}>{flag ? "true" : "false"}</button>     </div>   ); } 

Console:

App thinks that flag is false  _onClick thinks that flag is false App thinks that flag is true setTimeout thinks that flag is false  _onClick thinks that flag is true App thinks that flag is false setTimeout thinks that flag is true