Support extending classes

[mohebifar]

Edited by @nene:
The initial implementation has been merged in from #183, some things remain to be improved:

• We're currently overly strict by not detecting inheritance in the following code:

  function Class1() {}
  Class1.prototype = new Class2();

  we only detect the inheritance when the above is followed by constructor assignment:

  Class1.prototype.constructor = Class2;

• We detect importing util.inherits using require():

  var util = require("util");
  util.inherits(Class, SuperClass);

  but we'll fail with:

  import util from "util";
  util.inherits(Class, SuperClass);

• Call to superclass constructor must currently be directly inside the body of the constructor, the following will not work:

  function MyClass() {
    if (someCondition) {
      SuperClass.call(this);
    }
  }

• super() must be called before any references to this. The following code will fail:

  constructor() {
    this.age = 10; // ReferenceError
    super();
  }

• super() must always be called from subclass constructor. The following code will fail:

  class Foo extends Bar {
    constructor() {
      this.age = 10; // super() not called
    }
  }

• We should also convert superclass method calls inside other methods. Converting this:

  foo() {
    SuperClass.prototype.foo(this, params);
  }

  into this:

  foo() {
    super.foo(params);
  }

[vivinjoy]

+1

[vivinjoy]

@mohebifar , can we add transpiler dependent transforms for class inheritance? Since typescript implementation and coffeescript implementation of class inheritance differs slightly. I have created a transform for extending classes created via typescript transpiler. I could raise that PR.

[apexearth]

Do we want this added to the class transform or as an additional transform to be used in conjunction with the class transform?

[nene]

I think ideally it should be part of the class transform. But perhaps there are some practical considerations for not making it so.

[nene]

@apexearth The inheritance feature has now been finally released in 2.4.0. Should have released it sooner, but thought that I'll solve the two remaining small problems. But didn't really found time to tackle them.

[nene]

Closing this old task. Created #186 to handle the two remaining issues.