Memo

The memo function is a built-in utility for improving performance by reducing component re-renders.

Normally, when we instantiate a JSX element, like <MyComponent foo={bar} />, React will run our component function MyComponent with the props { foo: bar }. However, if our component function always returns the same thing given the same props, then React can memoize (save) the results of the previous call, and avoid calling the function again until props actually change.

Example

In this example, we'll see how wrapping a component function in memo limits the number of times the function is called.

Every time we press the button, our App function re-runs, since its state changed. This causes <Label /> to re-render, while <LabelMemo /> doesn't.

By default, props are compared for identity, essentially with ===. To change this behavior, we can pass an optional comparator function as the 2nd argument to memo. See the memo docs for more info.