Does React batch state update functions when using hooks?

Question

For class components, this.setState calls batch if inside event handlers. But what happens if state is updated outside the event handler and using useState hook?

function Component() {   const [a, setA] = useState('a');   const [b, setB] = useState('b');    function handleClick() {     Promise.resolve().then(() => {       setA('aa');       setB('bb');     });   }    return <button onClick={handleClick}>{a}-{b}</button> } 

Will it render aa - bb right away? Or it will be aa - b and then aa - bb?

Answer 1

TL;DR – if the state changes are triggered asynchronously (e.g. wrapped in a promise), they will not be batched; if they are triggered directly, they will be batched.

I've set up a sandbox to try this out: https://codesandbox.io/s/402pn5l989

import React, { Fragment, useState } from 'react'; import ReactDOM from 'react-dom';  import './styles.css';  function Component() {   const [a, setA] = useState('a');   const [b, setB] = useState('b');   console.log('a', a);   console.log('b', b);    function handleClickWithPromise() {     Promise.resolve().then(() => {       setA('aa');       setB('bb');     });   }    function handleClickWithoutPromise() {     setA('aa');     setB('bb');   }    return (     <Fragment>     <button onClick={handleClickWithPromise}>       {a}-{b} with promise     </button>     <button onClick={handleClickWithoutPromise}>       {a}-{b} without promise     </button>       </Fragment>   ); }  function App() {   return <Component />; }  const rootElement = document.getElementById('root'); ReactDOM.render(<App />, rootElement); 

I've made two buttons, one triggers the state changes wrapped in a promise like in your code example, the other triggers the state changes directly.

If you look at the console, when you hit the button “with promise”, it will first show a aa and b b, then a aa and b bb.

So the answer is no, in this case, it will not render aa - bb right away, each state change triggers a new render, there is no batching.

However, when you click the button “without promise”, the console will show a aa and b bb right away.

So in this case, React does batch the state changes and does one render for both together.