On Sat, Feb 15, 2014 at 3:17 AM, Anne van Kesteren <annevk at annevk.nl> wrote:
> It will take a long time before browsers support subclassing in
> general as far as I can tell.

I'm not talking about the class system in general.  I'm talking about
the ES5+ code:

```js
  Promise.prototype.bind = function(newThis) {
    var SuperPromise = this._bindSuper || this.constructor || Promise;
    // create a new Promise subclass (this is less cumbersome in es6, sigh)
    var BoundPromise = function(exec) {
      return SuperPromise.call(this, exec);
    };
    Object.setPrototypeOf(BoundPromise, SuperPromise);
    BoundPromise.prototype = Object.create(SuperPromise.prototype);
    BoundPromise.prototype.constructor = BoundPromise;
    BoundPromise.prototype._bindSuper = SuperPromise;

    BoundPromise.prototype.then = (function(superThen) {
      return function(f, r) {
        var ff = f && f.bind(newThis);
        var rr = r && r.bind(newThis);
        return superThen.call(this, ff, rr);
      };
    })(BoundPromise.prototype.then);
    return newThis ? BoundPromise.resolve(this) : SuperPromise.resolve(this);
  };
```
which is already useful in practice.  It would be nice if the browsers
would support Promise subclassing of this sort, in the (long?) interim
without ES6 class syntax.
  --scott

I'm not talking about the class system in general.  I'm talking about
the ES5+ code:

```js
  Promise.prototype.bind = function(newThis) {
    var SuperPromise = this._bindSuper || this.constructor || Promise;
    // create a new Promise subclass (this is less cumbersome in es6, sigh)
    var BoundPromise = function(exec) {
      return SuperPromise.call(this, exec);
    };
    Object.setPrototypeOf(BoundPromise, SuperPromise);
    BoundPromise.prototype = Object.create(SuperPromise.prototype);
    BoundPromise.prototype.constructor = BoundPromise;
    BoundPromise.prototype._bindSuper = SuperPromise;

    BoundPromise.prototype.then = (function(superThen) {
      return function(f, r) {
        var ff = f && f.bind(newThis);
        var rr = r && r.bind(newThis);
        return superThen.call(this, ff, rr);
      };
    })(BoundPromise.prototype.then);
    return newThis ? BoundPromise.resolve(this) : SuperPromise.resolve(this);
  };
```
which is already useful in practice.  It would be nice if the browsers
would support Promise subclassing of this sort, in the (long?) interim
without ES6 class syntax.