Bug: can't extend the native Map/WeakMap/Set/WeakSet class

[stuartZhang]

Bug: can't extend the native Map/WeakMap/Set/WeakSet class. Description: Only the ES 6 classes can't be extended. However, both the traditional built-in classes (e.g. Date) and the custom classes are able to be successfully extended to instantiate a sub class.

Source code:

class B extends Map{
  get b(){
    return 20;
  }
}
const b = new B();
console.log(b.b);

Run the JavaScript file by:

$ ../node_modules/.bin/traceur test.js

Runtime Error:

ModuleEvaluationError: Constructor Set requires 'new'
    at B.Set (native)
    at tailCall (/home/stzhang/var/webapps/connect/main/node_modules/traceur/bin/traceur.js:932:24)
    at Function.apply (/home/stzhang/var/webapps/connect/main/node_modules/traceur/bin/traceur.js:963:20)
    at new B (/home/stzhang/var/webapps/connect/main/unit_tests/mojioApiTokenRetrieval.bbjs:494:43)
    at /home/stzhang/var/webapps/connect/main/unit_tests/mojioApiTokenRetrieval.bbjs:500:11

[johnjbarton]

repl test case

https://google.github.io/traceur-compiler/demo/repl.html#class%20B%20extends%20Map%7B%0A%20%20get%20b()%7B%0A%20%20%20%20return%2020%3B%0A%20%20%7D%0A%7D%0Aconst%20b%20%3D%20new%20B()%3B%0Aconsole.log(b.b)%3B

[stuartZhang]

I suspect that the below statement throws an error that the "new" operator is required.

$traceurRuntime.superConstructor(B).apply(this, arguments);

In the ES6 program, the above corresponds to the statement "super();".

[stuartZhang]

Referencing the feedback from the previus issue: https://github.com/google/traceur-compiler/issues/1413, it works as design.

[arv]

There is a work around:

Compile:

class M extends Map {
  constructor() {
    super();
  }
}

to:

class M extends Map {
  constructor() {
    const self = new Map();
    self.__proto__ = M.prototype;  // should really be new.targe.prototype
    return self;
  }
}

but there are 2 issues making this hard:

1. Detecting when to do this transformation.
2. It needs new.target to work correctly and new.target is too expensive to emulate

[zloirock]

1. Simple list of built-in constructors by the spec. Isn't perfect, but better solutions will be much harder.
2. new.target.prototype -> Object.getPrototypeOf(this)

[arv]

You cannot touch this before calling super() in a constructor... but the compiled code is not using a constructor so it seems like it would work.

Another problem would be sub classing M... The way we compile constructors doesn't do the right thing when an object is returned.

[zloirock]

but the compiled code is not using a constructor so it seems like it would work.

Sure, it's approach for a compiled code, I use it for a long time.

Another problem would be sub classing M

Here required complete subclassing reform like in babel@6.