Functional component:

const handleFocus = (event) => event.target.select();
const Input = (props) => <input type="text" value="Some something" onFocus={handleFocus} />

ES6 class component:

class Input extends React.Component {
    handleFocus = (event) => event.target.select();

    render() {
        return (
            <input type="text" value="Some something" onFocus={this.handleFocus} />
        );
    }
}

React.createClass:

React.createClass({
    handleFocus: function(event) {
      event.target.select();
    },

    render: function() {
      return (
        <input type="text" value="Some something" onFocus={this.handleFocus} />
      );
    },
})