How do you properly return multiple values from a Promise?

Question

I've recently run into a certain situation a couple of times, which I didn't know how to solve properly. Assume the following code:

somethingAsync()   .then( afterSomething )   .then( afterSomethingElse )    function afterSomething( amazingData ) {   return processAsync( amazingData ); } function afterSomethingElse( processedData ) { } 

Now a situation might arise where I would want to have access to amazingData in afterSomethingElse.

One obvious solution would be to return an array or a hash from afterSomething, because, well, you can only return one value from a function. But I'm wondering if there is a way to have afterSomethingElse accept 2 parameters and invoke it likewise, as that seems a lot easier to document and understand.

I'm only wondering about this possibility since there is Q.spread, which does something similar to what I want.

Answer 1

You can't resolve a promise with multiple properties just like you can't return multiple values from a function. A promise conceptually represents a value over time so while you can represent composite values you can't put multiple values in a promise.

A promise inherently resolves with a single value - this is part of how Q works, how the Promises/A+ spec works and how the abstraction works.

The closest you can get is use Q.spread and return arrays or use ES6 destructuring if it's supported or you're willing to use a transpilation tool like BabelJS.

As for passing context down a promise chain please refer to Bergi's excellent canonical on that.

Answer 2

you can only pass one value, but it can be an array with multiples values within, as example:

function step1(){   let server = "myserver.com";   let data = "so much data, very impresive";   return Promise.resolve([server, data]); } 

on the other side, you can use the destructuring expression for ES2015 to get the individual values.

function step2([server, data]){   console.log(server); // print "myserver.com"   console.log(data);   // print "so much data, very impresive"   return Promise.resolve("done"); } 

to call both promise, chaining them:

step1() .then(step2) .then((msg)=>{   console.log(msg); // print "done" })