[javascript] How to set focus on an input field after rendering?

Focus on mount

If you just want to focus an element when it mounts (initially renders) a simple use of the autoFocus attribute will do.

<input type="text" autoFocus />

Dynamic focus

to control focus dynamically use a general function to hide implementation details from your components.

React 16.8 + Functional component - useFocus hook

const FocusDemo = () => {

    const [inputRef, setInputFocus] = useFocus()

    return (
        <> 
            <button onClick={setInputFocus} >
               FOCUS
            </button>
            <input ref={inputRef} />
        </>
    )
    
}

const useFocus = () => {
    const htmlElRef = useRef(null)
    const setFocus = () => {htmlElRef.current &&  htmlElRef.current.focus()}

    return [ htmlElRef, setFocus ] 
}

Full Demo

React 16.3 + Class Components - utilizeFocus

class App extends Component {
  constructor(props){
    super(props)
    this.inputFocus = utilizeFocus()
  }

  render(){
    return (
      <> 
          <button onClick={this.inputFocus.setFocus}>
             FOCUS
          </button>
          <input ref={this.inputFocus.ref}/>
      </>
    )
  } 
}
const utilizeFocus = () => {
    const ref = React.createRef()
    const setFocus = () => {ref.current &&  ref.current.focus()}

    return {setFocus, ref} 
}

Full Demo