directly

Like Promise.all, only less so

This module could more descriptively be named Promise.allButNotAllAtOnce. It takes an array of functions, each of which return a promise, and returns a promise which resolves once all those promises have resolved, or otherwise rejects... very similar to Promise.all. The difference is that a maximum of n promises are created at any one time. This is useful for rate-limiting asynchronous calls (e.g. fetch, mongoose...)

*** New feature *** Now supports throttling of potentially infinite queues of Promises (see notes on the Queue class below)

About the name

In the West Country people will often promise to do things 'directly' [drekt-lee], meaning they'll do it when they're good and ready, possibly never. Example usage:

I'll wash the dishes directly, my lover

Usage

A really common use case is as a drop-in replacement for Promise.all on a mapped array. Using arrow functions it's easy:

// before
Promise.all(myArray.map(myPromiseyFunction));

//after
directly(10, myArray.map(it => () => myPromiseyFunction(it)));

Other use cases

const directly = require('directly');
const urls = []; // a big array of urls
const fetchers = urls.map(function (url) {
    return function () {
        return fetch(url);
    }
});


directly(10, fetchers)
    .then(function (results) {
        // handle exactly as if it was a Promise.all()
    });

Can also be called as a constructor (in which case the .run() method should be used)

const Directly = require('Directly');
const throttledRequests = new Directly(10, fetchers)

throttledRequests
    .run()
    .then(function (results) {
        // handle exactly as if it was a Promise.all()
    })

 // can be used to stop the directly instance prematurely
throttledRequests.terminate()

To handle an infinite queue of promises use the Queue class to wrap your array of functions

fetchers = new directly.Queue(fetchers);
directly(10, fetchers)
    .catch(function (errorObject) {
        // You can handle any errors in here
        // The error object has 3 properties
        //  error: The error thrown
        //  nextError: A promise which will reject the next time an error is encountered
        //  terminate: A function to call which will terminate the directly instance
    });

// use push to add to the execution queue. Will work even if the queue has fallen idle
fetchers.push(func1, func2, func3)

Based on an idea originally developed at the FT