Use returned values from super calls as ‘this’

In ES2015, constructors which return an object implicitly substitute the value of this for any callers of super().As a result, it is necessary to capture any potential return value of super() and replace it with this.This change enables working with Custom Elements, which takes advantage of this to initialize browser-allocated elements with user-written constructors.

Example
  1. class Base {
  2.     x: number;
  3.     constructor() {
  4.         // return a new object other than `this`
  5.         return {
  6.             x: 1,
  7.         };
  8.     }
  9. }
  11. class Derived extends Base {
  12.     constructor() {
  13.         super();
  14.         this.x = 2;
  15.     }
  16. }

Generates:

  1. var Derived = (function (_super) {
  2.     __extends(Derived, _super);
  3.     function Derived() {
  4.         var _this = _super.call(this) || this;
  5.         _this.x = 2;
  6.         return _this;
  7.     }
  8.     return Derived;
  9. }(Base));

This change entails a break in the behavior of extending built-in classes like Error, Array, Map, etc.. Please see the extending built-ins breaking change documentation for more details.