I am searching for a long time on net. But no use. Please help or try to give some ideas how to achieve this. 

```js
var collection = {
        1245: {
          artist: "Robert Palmer",
          tracks: [ ]
        },
        5439: {
          album: "ABBA Gold"
        }
    };

    var collectionCopy = JSON.parse(JSON.stringify(collection));

    function update(id, prop, value) {
      if (value !== '' && prop == 'tracks') {
        collectionCopy[id][prop].push(value);

      return collectionCopy;
    }

    update(1245, "tracks", "Addicted to Love");
```

---

It says collectionCopy[id][prop].push(value) is not a function, i don't know why,but collectionCopy[id][prop] is actually an array.
Thanks for help!