I'm getting "Cannot read property 'score' of undefined", in my code I changed Player into a variable so that might be it

var PLAYERS = [
  {
    name: "Jim Hoskins",
    score: 31,
    id: 1,
  },
  {
    name: "Andrew Chalkley",
    score: 35,
    id: 2,
  },
  {
    name: "Alena Holligan",
    score: 42,
    id: 3,
  },
];

function Header(props) {
  return (
    <div className="header">
      <h1>{props.title}</h1>
    </div>
  );
}

Header.propTypes = {
  title: React.PropTypes.string.isRequired,
};

var Counter = React.createClass({
  propTypes: {
    initialScore: React.PropTypes.number.isRequired,
  },

  getInitialState: function() {
    return {
      score: 0,
    }
  },

  incrementScore: function(e) {
    this.setState({
      score: (this.state.score + 1),
    })
  },
  decrementScore: function(e) {
    this.setState({
      score: (this.state.score - 1),
    })
  },

  render: function() {
    return (
      <div className="counter">
        <button className="counter-action decrement" onClick={this.decrementScore}> - </button>
        <div className="counter-score"> {this.state.score} </div>
        <button className="counter-action increment" onClick={this.incrementScore}> + </button>
      </div>
    )
  }   
});  


var Player = React.createClass({
  propTypes: {
    name: React.PropTypes.string.isRequired,
    score: React.PropTypes.number.isRequired,
  },

  render: function(props) {
    return (
      <div className="player">
        <div className="player-name">
          {this.props.name}
        </div>
        <div className="player-score">
          <Counter initialScore={this.props.score} />
        </div>
      </div>
    )
  }  
});


function Application(props) {
  return (
    <div className="scoreboard">
      <Header title={props.title} />

      <div className="players">
        {props.players.map(function(player) {
          return <Player name={player.name} score={player.score} key={player.id} />
        })}
      </div>
    </div>
  );
}

Application.propTypes = {
  title: React.PropTypes.string,
  players: React.PropTypes.arrayOf(React.PropTypes.shape({
    name: React.PropTypes.string.isRequired,
    score: React.PropTypes.number.isRequired,
    id: React.PropTypes.number.isRequired,
  })).isRequired,
};

Application.defaultProps = {
  title: "Scoreboard",
}

ReactDOM.render(<Application players={PLAYERS}/>, document.getElementById('container'));