Promise.all().then() resolve?

Question

Using Node 4.x. When you have a Promise.all(promises).then() what is the proper way to resolve the data and pass it to the next .then()?

I

Answer 1

Today NodeJS supports new async/await syntax. This is an easy syntax and makes the life much easier

async function process(promises) { // must be an async function
    let x = await Promise.all(promises);  // now x will be an array
    x = x.map( tmp => tmp * 10);              // proccessing the data.
}

const promises = [
   new Promise(resolve => setTimeout(resolve, 0, 1)),
   new Promise(resolve => setTimeout(resolve, 0, 2))
];

process(promises)

Learn more:

• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function

Answer 2

But that doesn't seem like the proper way to do it..

That is indeed the proper way to do it (or at least a proper way to do it). This is a key aspect of promises, they're a pipeline, and the data can be massaged by the various handlers in the pipeline.

Example:

const promises = [
  new Promise(resolve => setTimeout(resolve, 0, 1)),
  new Promise(resolve => setTimeout(resolve, 0, 2))
];
Promise.all(promises)
  .then(data => {
    console.log("First handler", data);
    return data.map(entry => entry * 10);
  })
  .then(data => {
    console.log("Second handler", data);
  });

(catch handler omitted for brevity. In production code, always either propagate the promise, or handle rejection.)

The output we see from that is:

First handler [1,2]
Second handler [10,20]

...because the first handler gets the resolution of the two promises (1 and 2) as an array, and then creates a new array with each of those multiplied by 10 and returns it. The second handler gets what the first handler returned.

If the additional work you're doing is synchronous, you can also put it in the first handler:

Example:

const promises = [
  new Promise(resolve => setTimeout(resolve, 0, 1)),
  new Promise(resolve => setTimeout(resolve, 0, 2))
];
Promise.all(promises)
  .then(data => {
    console.log("Initial data", data);
    data = data.map(entry => entry * 10);
    console.log("Updated data", data);
    return data;
  });

...but if it's asynchronous you won't want to do that as it ends up getting nested, and the nesting can quickly get out of hand.

Answer 3

Your return data approach is correct, that's an example of promise chaining. If you return a promise from your .then() callback, JavaScript will resolve that promise and pass the data to the next then() callback.

Just be careful and make sure you handle errors with .catch(). Promise.all() rejects as soon as one of the promises in the array rejects.