Promise.race answers differently with same inputs, depending on when it is called.

ambiguous-race.js

// Use native or try any of these promise impls.
// They all behave the same.
//var Promise = require('when').Promise;
//var Promise = require('rsvp').Promise;
//var Promise = require('bluebird');

// Here are 2 promises, p1 and p2. p2 always *actually* finishes
// first, since p1 finishes in 1 millisecond, and p2 is already
// finished when created

var p1 = new Promise(function(resolve) {
	setTimeout(function() {
		resolve('p1');
	}, 1);
});

var p2 = Promise.resolve('p2');

// Given the same inputs, Promise.race answers differently
// depending only on when it is called.

// Logs p2
Promise.race([p1, p2]).then(console.log.bind(console));

// Logs p1
setTimeout(function() {
	// Same inputs, different outcome
	Promise.race([p1, p2]).then(console.log.bind(console));
}, 2);

After some discussion with other implementors and folks intimately involved with ES6 Promises, it seems that Promise.race is intended to mean: "wait for at least one promise to settle, then produce the outcome of the settled promise with the lowest iteration index."

The place this gets weird is when you apply Promise.race to an array containing more than 1 already-settled promises. IOW, it can't be used reliably to discover the promise that settled earliest in time, because, once-settled, a promise does not retain any notion of the time at which it settled.