Shallow memo is useful for components that take arrays/objects as props. If the reference check (===
) between two values fails, shallow-memo
will perform a shallow comparison if the values are both arrays or both objects.
Note: If a component doesn't take any arrays/objects as props, this isn't very useful.
npm install shallow-memo
import shallowMemo from "shallow-memo";
function MyComponent(props) {...};
const MemoizedMyComponent = React.memo(MyComponent, shallowMemo);
Inline objects/arrays create a new object/array every render. This means that while two objects or two arrays may represent the same values, they are not equal (using a ===
reference check).
This function will fall let you use inline objects/arrays without unnecessary re-renders.
// pass an inline object
ReactDOM.render((
<React.Fragment>
<MyComponent value={{ one: 1, two: 2 }} />
<MemoizedMyComponent value={{ one: 1, two: 2 }} />
</React.Fragment>
), root);
// re-rendering with the same values, but a different object
// the non-memoized component will re-render, but the memoized
// component will not
ReactDOM.render((
<React.Fragment>
<MyComponent value={{ one: 1, two: 2 }} />
<MemoizedMyComponent value={{ one: 1, two: 2 }} />
</React.Fragment>
), root);
This does not prevent re-renders when props contain React elements.
function Wrapper({ children }) {
return <div>{children}</div>;
}
// This is only useful when children is NOT a React element
const MemoizedWrapper = React.memo(Wrapper, shallowMemo);
// will NOT re-render
<MemoizedWrapper>
Hi!
</MemoizedWrapper>
// will NOT re-render
<MemoizedWrapper>
{"test"}{"ing"}
</MemoizedWrapper>
// will re-render
<MemoizedWrapper>
<Greeting value="Hi!" />
</MemoizedWrapper>
// will re-render
<MemoizedWrapper>
{"test"}<span>ing</span>
</MemoizedWrapper>