WeakSet

A WeakSet is a collection of garbage-collectable values, including objects and non-registered symbols. A value in the WeakSet may only occur once. It is unique in the WeakSet’s collection.

Values of WeakSets must be garbage-collectable. Most primitive data types can be arbitrarily created and don’t have a lifetime, so they cannot be stored. Objects and non-registered symbols can be stored because they are garbage-collectable.

The main differences to the Set object are:

WeakSets are collections of objects and symbols only. They cannot contain arbitrary values of any type, as Sets can. The WeakSet is weak, meaning references to objects in a WeakSet are held weakly. If no other references to a value stored in the WeakSet exist, those values can be garbage collected.