Never bypass the then method

[domenic]

Proposal

• Move the contents of the Then abstract operation into the Promise.prototype.then method definition.
• Everywhere we use the Then abstract operation, replace it with a call to the respective then method.
  • Promise.all
  • Promise.race
  • Promise.prototype.then's recursive case

Observable Changes

• This makes overriden thens govern the behavior of a promise, e.g.

  const p = new Promise(() => {});
  p.then = f => f(5);
  Promise.all([p]).then(([x]) => console.log(x)); // logs 5

• This forces implementations to make an observable call to then instead of using internal mechanisms. I imagine this will not actually be a burden.

• This does not impact Promise.resolve yet, i.e.

  const p = new Promise(r => r(1));
  p.then = f => f(2);
  Promise.resolve(p).then(x => console.log(x)); // still logs 1 right now

  for that, see Make Resolve operate the same on promises and thenables, using then #54.

[domenic]

@erights would appreciate your feedback before I implement this change; assigning to you.

[erights]

assigning to you

accepted. Need to think about this. I will point out that

p.then = ....

fails if p is frozen. If the assignment is in strict code, it fails noisily.