Mapromise

Promise.all([].map(() => {})).then(results => {}), serial! (or even concurent)

Usage

Serial

const Mapromise = require('mapromise');

const iterable = ['first', 'middle', 'last'];
function iterationCallback(value, index) {
	return new Promise((resolve) => {
		global.setTimeout(() => {
			resolve(value);
		}, (Math.random() * 10));
	});
}

Mapromise(iterable, iterationCallback).then(results => {
	console.log(results);
});

// > [first, middle, last]

Concurrent

const Mapromise = require('mapromise');

const iterable = ['first', 'middle', 'last'];
function iterationCallback(value, index) {
	return new Promise((resolve) => {
		global.setTimeout(() => {
			resolve(value);
		}, (Math.random() * 10));
	});
}

Mapromise(Iterable, iterationCallback, {concurency: 50}).then(results => {
	console.log(results);
});

// > [first, middle, last], order of execution is not guaranteed, but order of results is

In

• Iterable collection
• Iterator callback - Asynchronous callback invoked on each value of the iterable
• options defaults being:

{
   collect: true, // Do you want to collect results of each iteration (Array.map behavior)
   concurrency: 1 // For the concurrent variant
}

Out

Promise, duh!

• Resolves to either array of results (ordered), or number of iterations (not index)
• Rejects with the first rejection reason in series. On rejection, no subsequent callbacks are invoked

Only arrays?

Everything that implements iterator interface (has next method), or has Symbol.iterator method defined.

But why?

Because I had some uses for this behavior. Async.mapSeries is the best comparison. Promises didn't offer this functionality natively and I don't know if Bluebird does.

Tests?

There are some. Though, they are mostly just pure smoke tests and slight torture/heap-explosion ones.

What is perf.js ?

index.js is the simplest implementation of mapping promises using async/await I came up with, but it has one problem. It starts to suck on higher concurrency numbers. That is caused by holding promise references and await Promise.all[] which sucks up a lot of memory compared to more complex implementation using some kind of defered resolution. perf.js uses a single defered promise to manage iteration state and is much more memory efficient on higher concurrency numbers. See perf-test