Record Collection // I don't understand given answer

JavaScript
1

Tell us what’s happening:
Hey, this challenge was too hard for me, so I had a look at the solution from the hints section.
The problem is I still don’t understand one or two things, which might be correlated.

  1. The instruction says: “If prop is “tracks” but the album doesn’t have a “tracks” property, create an empty array before adding the new value to the album’s corresponding property.”

I can’t see it in the code. Where is the array created before adding the value?

  1. Why in the code at some point is:

collection[id][prop]= [value]

with the value in brackets and why the code won’t work without them?

Your code so far


// Setup
var collection = {
    "2548": {
      "album": "Slippery When Wet",
      "artist": "Bon Jovi",
      "tracks": [ 
        "Let It Rock", 
        "You Give Love a Bad Name" 
      ]
    },
    "2468": {
      "album": "1999",
      "artist": "Prince",
      "tracks": [ 
        "1999", 
        "Little Red Corvette" 
      ]
    },
    "1245": {
      "artist": "Robert Palmer",
      "tracks": [ ]
    },
    "5439": {
      "album": "ABBA Gold"
    }
};
// Keep a copy of the collection for tests
var collectionCopy = JSON.parse(JSON.stringify(collection));

// Only change code below this line
function updateRecords(id, prop, value) {
  if (prop === "tracks" && value !== "") {
   if(collection[id][prop]) {
    collection[id][prop].push(value);
   }
   else {
    collection[id][prop]= [value];
   }
  } else if (value !== "") {
    collection[id][prop] = value;
  } else {
    delete collection[id][prop];
  }

  return collection;
}

// Alter values below to test your code
updateRecords(5439, "artist", "ABBA");

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Link to the challenge: