ECMAScript 6: what is WeakSet for?

Question

The WeakSet is supposed to store elements by weak reference. That is, if an object is not referenced by anything else, it should be cleaned from the WeakSet.

I have

Answer 1

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)