If a parent component is updated, does React always update all the direct children within that component?
No. React will only re-render a component if shouldComponentUpdate()
returns true
. By default, that method always returns true
to avoid any subtle bugs for newcomers (and as William B pointed out, the DOM won’t actually update unless something changed, lowering the impact).
To prevent your sub-component from re-rendering unnecessarily, you need to implement the shouldComponentUpdate
method in such a way that it only returns true
when the data has actually changed. If this.props.messages
is always the same array, it could be as simple as this:
shouldComponentUpdate(nextProps) {
return (this.props.messages !== nextProps.messages);
}
You may also want to do some sort of deep comparison or comparison of the message IDs or something, it depends on your requirements.
EDIT: After a few years many people are using functional components. If that’s the case for you then you’ll want to check out React.memo. By default functional components will re-render every time just like the default behavior of class components. To modify that behavior you can use React.memo()
and optionally provide an areEqual()
function.