Set.prototype.difference, simplified

difference.md

Context: tc39/proposal-set-methods#50

Proposed spec texts

Set.prototype.difference(arg), minimalist

1. If Type(this) is not Object, or this does not have a [[SetData]] internal slot, throw a new TypeError exception.
2. If Type(arg) is not Object, or arg does not have a [[SetData]] internal slot, throw a new TypeError exception.
3. Let diffList be a List containing all elements that are in this.[[SetData]] but are not in arg.[[SetData]], using SameValueZero to determine identity. (Optional: formalize this with a loop that operates on the Lists.)
4. Let newSet be ? OrdinaryCreateFromConstructor(%Set%, "%SetPrototype%", « [[SetData]] »).
5. Set newSet.[[SetData]] to diffList.
6. Return newSet.

If we wanted to accept iterables as arguments, then we would do the following:

Set.prototype.difference(arg), supports iterables

1. If Type(this) is not Object, or this does not have a [[SetData]] internal slot, throw a new TypeError exception.
2. Let argList be the empty list.
3. If arg is not undefined or null, then set argList to ? IterableToList(arg).
4. Let diffList be a List containing all elements that are in this.[[SetData]] but are not in argList, using SameValueZero to determine identity. (Optional: formalize this with a loop that operates on the Lists.)
5. Let newSet be ? OrdinaryCreateFromConstructor(%Set%, "%SetPrototype%", « [[SetData]] »).
6. Set newSet.[[SetData]] to diffList.
7. Return newSet.