React: How to Child component re-render only prop update

Question

I have:

function Parent(){
  const [num, setNum] = useState(0)
  const [someState, setSomeState] = useState({})
  return <div>
      <Child num={num}/>
    </div>
}

function Child({num}) {
  return <div>{num}</div>
}

I want Child re-render only num is updated. If only someState update, Child won't re-render.

Answer 1

You can use React.memo and wrap your children component which you want to re-render only when props are changed.

Example:

import {useState, memo} from 'react';
import "./styles.css";

export default function App() {
  const [num, setNum] = useState(0)
  const [someState, setSomeState] = useState({})

  return (
    <div className="App">
      <h1>Hello CodeSandbox</h1>
      <h2>Start editing to see some magic happen!</h2>
      <button onClick={() => setSomeState({id: 1})}>Change state</button>
      <button onClick={() => setNum(num + 1)}>Change num</button>
      <Child num={num}/>
    </div>
  );
}

const Child = memo(function Child({num}) {
  console.log('child render');
  return <div>{num}</div>
});