在Promise.prototype.then中将promise作为onFulfilled函数传递

Question

I recently discovered that you can pass a promise, instead of a function as onFulfilled function in promise.then

For example:

 var p = new Promise(function(resolve, reject) { setTimeout(function(){ resolve("First Promise"); }, 4000); }); var q = new Promise(function(resolve, reject) { setTimeout(function(){ console.log("Yo"); resolve("Second Promise"); }, 8000); }); p.then(q).then(function(data){ console.log(data); }); 

This will print the following output

First Promise (at 4th second)
Yo (at 8th second)

This sounded weird to me, because as practise, we only pass functions in promise.then , instead of passing a new Promise.

Can someone help me certain use case, where it can be used? And, what is the exact behaviour, if you pass promise inside promise.then ?

EDIT:

I saw similar behaviour on Spotify Web Player.

this._onStreamerConnect() returns a Promise, which has .then attached on it. .then takes a parameter e which is also a Promise, as seen from the Watch tab in the right panel

Answer 1

According to MDN documentation :

If one or both arguments are omitted or are provided non-functions, then then will be missing the handler(s), but will not generate any errors. If the Promise that then is called on adopts a state ( fulfillment or rejection ) for which then has no handler, a new Promise is created with no additional handlers, simply adopting the final state of the original Promise on which then was called.

This means that calling p.then(q).then(function...) is the same as calling p.then(function...) . The q parameter is completely ignored and not part of the promise chain.

q 's executor function eventually resolves in 8 seconds, independent of p , which still resolves in 4 seconds, and nothing consumes the string "Second Promise" as you can see from the output.

Answer 2

To add to Patrick Roberts's correct answer , you can return a Promise from then , and the then function will wait for the Promise before continuing . However, to do that, you need to pass a function to then that returns a Promise .

As such, if your line were to look like this:

p.then(() => q).then(function(data){
    console.log(data);
});

Or without lambdas:

p.then(function() { return q; }).then(function(data){
    console.log(data);
});

Then you would see the output:

First Promise (at 4th second)
Yo (at 8th second)
Second Promise (at 8th second)

---

EDIT : To your point about Spotify, most calls to the _runOnDevice function you're looking at have explicitly-bound functions as their second and third arguments.

Most of the argument names are the same ( t , e , n ) despite their positions, which makes it a bit confusing to follow. I'd guess that these are transpiled async functions heavily using lambdas.

As Patrick pointed out in the comments, at least one call to _runOnDevice appears to take a Promise as a second parameter. That looks like a bug to me.