1unique_key.js

var UniqueKeyContainer = React.createClass({
  render() {
    var someContents = this.props.listOfStuff.map((stuff) => (
      <li key={stuff.id}>{stuff.title}</li>
    ));
    
    return (
      <ul>
        {someContents}
      </ul>
    );
  }
});

2index_keys.js

var UniqueKeyContainer = React.createClass({
  render() {
    var someContents = this.props.listOfStuff.map((stuff, index) => (
      <li key={index}>{stuff.title}</li>
    ));
    
    return (
      <ul>
        {someContents}
      </ul>
    );
  }
});