The ES2015 WeakMap and WeakSet classes

Along with the two new data structures Set and Map, ES2015 also introduced a weak type version of these classes: WeakMap and WeakSet.

Basically, the only difference between the Map or Set classes and their weak versions are:

• The WeakSet or WeakMap classes do not have the entries, keys, and values methods
• It is only possible to use objects as keys

The reason for creating and using these two classes is performance-related. Since WeakSet and WeakMap are weakly typed (using the object as a key), there is no strong reference to the keys. This behavior allows the JavaScript garbage collector to clean an entire entry from the map or set.

Another advantage of the weak versions is we can only retrieve a value if you have its key. Since these classes do not have the iterator methods (entries, keys, and values), there is no way to retrieve a value unless you know what the key is. This confirms our choice in using the WeakMap class to encapsulate the private properties...