props.js

var ComponentWithDefaultProps = React.createClass({
  getDefaultProps: function() {
    return {
      value: 'default value'
    };
}
});

------------------ or the other way  ----------------------

ComponentWithDefaultProps. defaultProps = {
value:'default value',
}

class ComponentWithDefaultProps extends Component {
render(){
return(
 <h1>{this.props.val}</h1>
          );
}
}