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JavaScript

var arrayOfSongs = [{
  "title": "Blue",
  "duration": 161.71,
  "audioUrl": "/assets/music/blue",
  "playing": false,
  "playedAt": "2016-12-21T22:58:55.203Z"
}, {
  "title": "Blue",
  "duration": 161.71,
  "audioUrl": "/assets/music/blue",
  "playing": false,
  "playedAt": "2016-12-21T22:58:55.203Z"
}, {
  "title": "Blue",
  "duration": 161.71,
  "audioUrl": "/assets/music/blue",
  "playing": false,
  "playedAt": "2016-12-21T22:58:55.203Z"
}, {
  "title": "Green",
  "duration": 161.71,
  "audioUrl": "/assets/music/blue",
  "playing": false,
  "playedAt": "2016-12-21T22:58:55.203Z"
}];

function getPlayCount(arrayOfSongs) {
  let songObj = {};
  let SongSet = new Set();
  arrayOfSongs.map(obj => (SongSet.has(obj.title)) ? true : SongSet.add(obj.title));
  for (let songTitle of SongSet.values()) {
    songObj[songTitle] = {
      playCount: 0
    };
    arrayOfSongs.map(obj => (obj.title === songTitle) ? songObj[songTitle].playCount++ : false)
  }
  return songObj;
}

console.log(getPlayCount(arrayOfSongs));