Can't understand how property was added to object

I don’t understand how property ‘artist’ was added to ‘5439’ object. Please explain or give a link where I can read explanation.

const collection = {
  5439: {
    album: 'ABBA Gold',

function updateRecords(id, prop, value) {
  if (prop === 'tracks' && value !== '') {

  } else if (value !== '') {
    collection[id][prop] = value;
  return collection;

updateRecords(5439, 'artist', 'ABBA');,console

The object collection has a key of 5439 (the id). That has a value of {album: 'ABBA Gold'}.

You are saying collection[id][prop] = value, so collection[5439]['artist'] = 'ABBA' so assigning the value 'ABBA' to a key called ‘artist’ on the object with the key 5439 on the object collection

collection = {
  5439: {
    artist: 'ABBA',
    album: 'ABBA Gold'

collection[5439]['artist'] is ABBA, collection[5439]['album'] is ABBA Gold. If you do

collection[5439]['artist'] = 'The Beatles'

It’ll change artist to the Beatles. If you do

collection[5439]['favouritePet'] = 'Cat'

It’ll add a ‘favouritePet’ property and so on.

I’ve tested this behavior and now I understand that the property will be added automatically if there is none. Thank you.