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| type props = { | |
| callback: (a: string) => void; | |
| } | |
| const Child: React.FC<props> = ({callback}) => { | |
| const [value, setVlaue] = useState<number>(0); | |
| const isEven = () => { | |
| let i = 0; | |
| // this while loop is to mock a costly operation | |
| while(i<1000000000) i++; | |
| callback('isEven called'); | |
| return value % 2 === 0; | |
| } | |
| return ( | |
| <div> | |
| { | |
| isEven() ? 'even': 'odd' | |
| } | |
| <button onClick={() => setVlaue(prev => prev + 1)}>Add one</button> | |
| </div> | |
| ); | |
| }; | |
| export default React.memo(Child); |
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