Promise.all() will have unexpected behavior if Array.prototype.then exists and is callable (?)

[tc39]

Promise.all() will have unexpected behavior if Array.prototype.then exists and is callable (?)

[domenic]

Nope, this is working as intended. You should never be able to fulfill with a thenable; that's why resolving with a thenable triggers its then function.

[domenic]

An analogous situation: if you remove [Symbol.iterator] from Array.prototype, then passing an array in to Promise.all will not work as you expect.

[smikes]

Although I see your point, I believe this behavior will be surprising to many -- similar to Promise.race([]) returning a never-fulfilled promise; it makes sense, but it isn't expected.

In particular, the difference between deleting Symbol.iterator and adding Array.prototype.then is that deleting Symbol.iterator is obviously bad, and will visibly break lots of syntax; while adding a 'then' to a object prototype will only affect Promise.all.

Psychology plays into this as well. It's easier to understand that removing something causes damage than adding something seemingly innocuous which happens to have a magic name.

Consider:

var p1 = Promise.resolve(1);
var p2 = Promise.resolve(2);
var logger = console.log.bind(console);

Promise.all([p1, p2]).then(logger); // outputs [1, 2]

Array.prototype.then = function (a, b) { a('foo'); }

Promise.all([p1, p2]).then(logger); // outputs foo

Or how about this:

Object.prototype.then = function (a, b) { a('baz'); }

Promise.resolve({}).then(logger);  // outputs baz

I think that's pretty surprising. Especially if the definition of then comes from loading a library or some other code.

This behavior is visible in Chrome 38.0.2125.104 m

[smikes]

i thought of two more:

O.p.then = function (a, b) { b(new SyntaxError("illegal character")); }

O.p.then = function () {} // all promises fulfilled with an object will pend forever

And another:

O.p.then = function (a) { console.log(JSON.stringify(this)); a(this); } // log all promise fulfillment args

[domenic]

Shrug. This is just how promises work. then has essentially become a reserved property name.

[smikes]

OK. I will add tests to cover this behavior.