Focus Management

Helping the user's focus stay on the right place is a key part of the user experience. This is especially important for users who rely on screen readers or keyboard navigation. But even able users can benefit from a well-thought focus management experience.

Sometimes, the element you want to focus on only becomes available after a state update. For example:

function MyComponent() {
	const [show, setShow] = useState(false)

	return (
		<div>
			<button onClick={() => setShow(true)}>Show</button>
			{show ? <input /> : null}
		</div>
	)
}

Presumably after the user clicks "show" they will want to type something in the input there. Good focus management would focus the input after it becomes visible.

It's important for you to know that in React state updates happen in batches. So state updates do not necessarily take place at the same time you call the state updater function.

As a result of React state update batching, if you try to focus an element right after a state update, it might not work as expected. This is because the element you want to focus on might not be available yet.

function MyComponent() {
	const inputRef = useRef<HTMLInputElement>(null)
	const [show, setShow] = useState(false)

	return (
		<div>
			<button
				onClick={() => {
					setShow(true)
					inputRef.current?.focus() // This probably won't work
				}}
			>
				Show
			</button>
			{show ? <input ref={inputRef} /> : null}
		</div>
	)
}

The solution to this problem is to force React to run the state and DOM updates synchronously so that the element you want to focus on is available when you try to focus it.

You do this by using the flushSync function from the react-dom package.

import { flushSync } from 'react-dom'

function MyComponent() {
	const inputRef = useRef<HTMLInputElement>(null)
	const [show, setShow] = useState(false)

	return (
		<div>
			<button
				onClick={() => {
					flushSync(() => {
						setShow(true)
					})
					inputRef.current?.focus()
				}}
			>
				Show
			</button>
			{show ? <input ref={inputRef} /> : null}
		</div>
	)
}

What flushSync does is that it forces React to run the state update and DOM update synchronously. This way, the input element will be available when you try to focus it on the line following the flushSync call.

In general you want to avoid this de-optimization, but in some cases (like focus management), it's the perfect solution.

Learn more in 📜 the flushSync docs.