FocusWithin is a component that allows detecting if one of its children has focus. It can be considered as a missing JS API for focus-within.

FocusWithin will fire onFocus once one of its children will receive focus. Similarly onBlur is going to be fired once focus is left all its children.

Simple example

Open developer console to see log messages.

<FocusWithin
  onFocus={() => {
    console.log('Received focus')
  }}
  onBlur={() => {
    console.log('Lost focus')
  }}
>
  <input type="text" placeholder="Click to activate first input" />
  <input type="text" placeholder="Use Tab to activate next input" />
  <button>A button to try focus</button>
</FocusWithin>

Reacting to the focus change

If you want to react to the focus change, use function as a children pattern. When function is used as children, you must provide the ref prop.

<FocusWithin
  onFocus={() => {
    console.log('Received focus')
  }}
  onBlur={() => {
    console.log('Lost focus')
  }}
>
  {({ focused, getRef }) => (
    <form>
      <fieldset
        ref={getRef}
        style={{ borderColor: focused ? 'blue' : '#999' }}
      >
        <label htmlFor="input1">First input</label>
        <input
          id="input1"
          type="text"
          placeholder="Click to activate first input"
        />
        <label htmlFor="input2">First input</label>
        <input
          id="input2"
          type="text"
          placeholder="Use Tab to activate next input"
        />
        <button type="submit">Submit</button>
        <p style={{ color: focused ? 'danger' : 'text' }}>
          {focused ? 'Focus is inside' : 'No focus here'}
        </p>
      </fieldset>
    </form>
  )}
</FocusWithin>

Using with CSS-in-JS libs

If you're using a CSS-in-JS library like styled-components you need to pass a ref using ref prop. You can use getRef function from the parameters.

;({ focused: Boolean, getRef: Function }) => React.Element

const styled = require('styled-components').default
const StyledBox = styled('div')`
  padding: 20px;
  border: 1px solid;
  border-color: ${props =>
    props.focused ? 'palevioletred' : '#999'};

  & > * + * {
    margin-left: 20px;
  }
`
;<FocusWithin
  onFocus={() => {
    console.log('Received focus')
  }}
  onBlur={() => {
    console.log('Lost focus')
  }}
>
  {({ focused, getRef }) => (
    <StyledBox ref={getRef} focused={focused}>
      <input
        type="text"
        placeholder="Click to activate first input"
      />
      <input
        type="text"
        placeholder="Use Tab to activate next input"
      />
      <button>A button to try focus</button>
    </StyledBox>
  )}
</FocusWithin>

Note: It's recommended to use :focus-within selector instead of interpoaltions whenever possible.

Focus method

Sometimes it's needed to focus the container node programmatically. You can use the public method focus. Note that tabIndex={-1} needs to be set on non-interactive elements to make them receive focus.

const ref = React.createRef()
;<div>
  <FocusWithin ref={ref}>
    {({ focused, getRef }) => (
      <span tabIndex={-1} ref={getRef}>
        {focused ? 'Focused' : 'Not focused'}
      </span>
    )}
  </FocusWithin>
  <button
    onClick={() => {
      ref.current.focus()
    }}
  >
    Focus the span
  </button>
</div>

Naïve focus trap implementation

import { useRef, useState, useEffect } from 'react'

const firstInput = useRef(null)
const [enabled, setEnabled] = useState(false)

useEffect(() => {
  if (enabled) {
    firstInput.current.focus()
  }
}, [enabled])
;<FocusWithin
  onBlur={() => {
    enabled && firstInput.current.focus()
  }}
>
  <fieldset>
    <input
      type="text"
      placeholder="Click to activate first input"
      ref={firstInput}
    />
    <input type="text" placeholder="Use Tab to activate next input" />
    <button
      type="submit"
      onClick={() => {
        setEnabled(!enabled)
      }}
    >
      {enabled ? 'Disable' : 'Enable'} focus trap
    </button>
  </fieldset>
</FocusWithin>