Overwrite non-subclassable promises

[jakearchibald]

Firefox/Chrome promises throw when you call constructor on a subclass. The polyfill should take over in browsers that do that.

[stefanpenner]

👍

[stefanpenner]

so currently precisely 0 browser support this. Would we like to still check and override?

[jakearchibald]

Happy to clobber all until we get an implementation that supports subclassing, then we can look at how best to feature test it.

[stefanpenner]

• subclass-ability is now a criteria for polyfil
• update readme accordingly

Something like the following (although it adds many bytes).

var isNative = Object.prototype.toString.call(Promise.resolve())  === '[object Promise]';

function testSubclassing() {
  function SubClass() {
    Promise.call(this);
  }

  SubClass.prototype= Object.create(Promise.prototype)

  var seemsToSubClass;
  try {
    new SubClass(function() { });
    seemsToSubClass  = true;
  } catch(e) {
    seemsToSubClass = false
  }

  return seemsToSubClass;
}



if (isNative) {
  canSubclass = testSubClassing();
}

[stefanpenner]

master should do this now, but If you have time to audit it that would be awesome.!