Ember.Map Class packages/ember-metal/lib/map.js:226

A Map stores values indexed by keys. Unlike JavaScript's default Objects, the keys of a Map can be any JavaScript object.

Internally, a Map has two data structures:

1. keys: an OrderedSet of all of the existing keys
2. values: a JavaScript Object indexed by the Ember.guidFor(key)

When a key/value pair is added for the first time, we add the key to the keys OrderedSet, and create or replace an entry in values. When an entry is deleted, we delete its entry in keys and values.