React: How to navigate through list by arrow keys

Try something like this:

export default class Example extends Component {
  constructor(props) {
    super(props)
    this.handleKeyDown = this.handleKeyDown.bind(this)
    this.state = {
      cursor: 0,
      result: []
    }
  }

  handleKeyDown(e) {
    const { cursor, result } = this.state
    // arrow up/down button should select next/previous list element
    if (e.keyCode === 38 && cursor > 0) {
      this.setState( prevState => ({
        cursor: prevState.cursor - 1
      }))
    } else if (e.keyCode === 40 && cursor < result.length - 1) {
      this.setState( prevState => ({
        cursor: prevState.cursor + 1
      }))
    }
  }

  render() {
    const { cursor } = this.state

    return (
      <Container>
        <Input onKeyDown={ this.handleKeyDown }/>
        <List>
          {
            result.map((item, i) => (
              <List.Item
                key={ item._id }
                className={cursor === i ? 'active' : null}
              >
                <span>{ item.title }</span>
              </List.Item>
            ))
          }
        </List>
      </Container>
    )
  }
}

The cursor keeps track of your position in the list, so when the user presses the up or down arrow key you decrement/increment the cursor accordingly. The cursor should coincide with the array indices.

You probably want onKeyDown for watching the arrow keys instead of onChange, so you don’t have a delay or mess with your standard input editing behavior.

In your render loop you just check the index against the cursor to see which one is active.

If you are filtering the result set based on the input from the field, you can just reset your cursor to zero anytime you filter the set so you can always keep the behavior consistent.