If you use yield in JS you don’t have to think about execition order too. Because execution sequence will always be the same, and JS lacks real threading, so you won’t notice it anyway.
var x;
promise().then(function (result) {
x = result;
}).done();
x.doStuff();
This is a contrived example, and pretty stupid too. Still, if promises are always resolved on next tick, it will always fail. If promises may be resolved in the same tick, then you may miss the mistake.
A more real-world example is synchronization code like this[1]. Can you say at a glance if this code works fine both with resolved and pending promises? You'd have to check every `then`. You don't get this kind of problem with always-asynchronous promises.
In other words, it's a tradeoff between performance and introducing possibility of several different code paths, and it gets messier with non-trivial code.
