Let’s see how to set focus on input after a component is rendered. Use componentDidMount and refs callback to focus on the element.

componentDidMount(){
  
 this.nameInput.focus(); 
}

Use it as shown below:

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'));



<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.3.1/react.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.3.1/react-dom.js"></script>
<div id="app"></div>

This method works but there is a chance that it will not work if your component renders nothing first. You can solve this problem by adding this.refs.nameInput.getDOMNode.focus(); in componentDidMount. This problem is because the element is already mounted. You can also use

autoFocus when mounted.

<input autoFocus name=...

The setback is that it focuses input on any re-render which might not be desired.