On 8/22/2013 8:04 AM, Mark S. Miller wrote:
>     var Promise = (function(){
>         "use strict"; // of course
>
>         var brand = new WeakMap();
>
>         // only ever called with "new"
>         function HiddenPromiseConstructor(callback) {
>             // initialize "this", which it can assume starts fresh and 
> trustworthily uninitialized
>             brand.set(this, true);
>         }
>
>         function Promise(arg) {
>             if (Object.getPrototypeOf(this) === Promise.prototype && 
> !(brand.has(this))) {
>                 // assume likely called with "new", but do not trust 
> "this"
>                 return new HiddenPromiseConstructor(arg)
>             } else {
>                 // assume called for coercion behavior. Ignore this
>                 if (brand.has(arg)) {
>                     return arg;
>                 } else {
>                     return Promise.of(arg);
>                 }
>             }
>         }
>         HiddenPromiseConstructor.prototype = Promise.prototype;
>
>         // initialize Promise.prototype
>         // initialize Promise statics
>         return Promise
>     })();

I'd make one small change to this:

     if (Object.getPrototypeOf(this) === Promise.prototype && 
!(brand.has(this))) {

to

     if (this instanceof Promise && !(brand.has(this))) {

or an alternative that I think is functionally identical, if you want to 
avoid instanceof:

     if (Promise.prototype.isPrototypeOf(this) && !(brand.has(this))) {


This change would allow subclassing of Promise.

I'd make one small change to this:

```js
if (Object.getPrototypeOf(this) === Promise.prototype && !(brand.has(this))) {
```

to

```js
if (this instanceof Promise && !(brand.has(this))) {
```

or an alternative that I think is functionally identical, if you want to 
avoid instanceof:

```js
if (Promise.prototype.isPrototypeOf(this) && !(brand.has(this))) {
```

This change would allow subclassing of Promise.