How to focus an input element on page load?

You can do it by creating ref for input element and using it in componentDidMount():

class App extends React.Component {
  componentDidMount() {
    this.nameInput.focus();
  }
  render() {
    return (
      <div>
        <input defaultValue={"Won't focus"} />
        <input ref={(input) => (this.nameInput = input)} defaultValue={'Will focus'} />
      </div>
    );
  }
}
ReactDOM.render(<App />, document.getElementById('app'));