This is a really hard question. To be completely honest I had no idea in the context of JavaScript so I asked in esdiscuss and got a convincing answer from Domenic.
WeakSets are useful for security and validation reasons. If you want to be able to isolate a piece of JavaScript. They allow you to tag an object to indicate it belongs to a special set of object.
Let's say I have a class ApiRequest:
class ApiRequest {
constructor() {
// bring object to a consistent state, use platform code you have no direct access to
}
makeRequest() {
// do work
}
}
Now, I'm writing a JavaScript platform - my platform allows you to run JavaScript to make calls - to make those calls you need a ApiRequest - I only want you to make ApiRequests with the objects I give you so you can't bypass any constraints I have in place.
However, at the moment nothing is stopping you from doing:
ApiRequest.prototype.makeRequest.call(null, args); // make request as function
Object.create(ApiRequest.prototype).makeRequest(); // no initialization
function Foo(){}; Foo.prototype = ApiRequest.prototype; new Foo().makeRequest(); // no super
And so on, note that you can't keep a normal list or array of ApiRequest objects since that would prevent them from being garbage collected. Other than a closure, anything can be achieved with public methods like Object.getOwnPropertyNames or Object.getOwnSymbols. So you one up me and do:
const requests = new WeakSet();
class ApiRequest {
constructor() {
requests.add(this);
}
makeRequest() {
if(!request.has(this)) throw new Error("Invalid access");
// do work
}
}
Now, no matter what I do - I must hold a valid ApiRequest object to call the makeRequest method on it. This is impossible without a WeakMap/WeakSet.
So in short - WeakMaps are useful for writing platforms in JavaScript. Normally this sort of validation is done on the C++ side but adding these features will enable moving and making things in JavaScript.
(Of course, everything a WeakSet does a WeakMap that maps values to true can also do, but that's true for any map/set construct)
(Like Bergi's answer suggests, there is never a reason to add an object literal directly to a WeakMap or a WeakSet)
