Optimization

So you have a working app and want to decrease unneccessary renders and loads

• Look at using more granular contexts, Redux with selectors, or a custom hook that accesses local storage

• React.lazy load with Suspense

React.memo

If a parent component changes state/rerenders, all of its child also rerender unless you use React.memo. The DOM might not change though.

Use React.memo when given the same props, component returns same output and reasonably expensive/used with same props. No internal state?

Like shouldComponentUpdate of old, React.memo shallowly compare its props before deciding to rerender:

const Button = React.memo((props) => {
  // your component
});

React.memo is equivalent to PureComponent, but it only compares props. (2nd arg can be custom compare ft, that skips rerender if returns true)

Can optimize individual children with useMemo.

// Only re-rendered if `a` changes:
const child1 = useMemo(() => <Child1 a={a} />, [a]);

return (<>{child1}</>);

React only takes React.memo as a hint, must be allowed to rerender