Working Asynchronously with async in Node.js

As mentioned previously, Node.js runs in a single thread. In order to get it running with concurrency you need to use a library. In today’s post, I’m going to go through the async library really briefly.

It’s important to note that you need to follow conventions in order for this library to be successful for you.

All these functions assume you follow the Node.js convention of providing a single callback as the last argument of your async function.

List processing

A great deal of asynchronous work that you’ll do will be conducted on collections/lists. The async module provides the usual processing facilities for these data types, and they’re all simple to use. Here we’ll filter out non-prime numbers:

var async = require('async'),
    range = require('node-range');

var candidates = range(3, 1000).toArray();

var isPrime = function (v, callback) {

  if ((v % 2) == 0) {
    callback(false);
    return;
  }

  var m = 3;

  while (m < v) {
    if ((v % m) == 0) { 
      callback(false);
      return;
    }

    m += 2;
  }

  callback(true);
};


async.filter(candidates, isPrime, function (res) {
  console.log(res);
});

Note that isPrime uses a callback to send its result back. This allows all items in the array, candidates to participate nicely in the async operation.

Work sequencing

There are a few different work strategies you can employ with the async module.

series will execute items one-after-the-other; parallel execute items at the same time (or, in parallel); waterfall operates like series however it’ll automatically supply the return of the previous call as input to the next.

Everything you expect is in this library.