Ember.MapWithDefault Class packages/ember-metal/lib/map.js:423

Removes a value from the map for an associated key.

Iterate over all the keys and values. Calls the function once for each key, passing in value, key, and the map being iterated over, in that order.

The keys are guaranteed to be iterated over in insertion order.

Check whether a key is present.

Adds a value to the map. If a value for the given key has already been provided, the new value will replace the old value.