Node.js - Async.js: how does parallel execution work?

Problem

I want to know how parallel execution works in async.js

async = require('async')

async.parallel([
    function(callback){
        for (var i = 0; i < 1000000000; i++) /* Do nothing */;
        console.log("function: 1")
    },
    function(callback){
        console.log("function: 2")
    }
]);

In the above example, I expect obtain the output:

function: 2

function: 1

but, the console throws the inverse, what is happening? thanks.

Problem courtesy of: Danilo Aburto Vivians

Solution

You get the answer you don't expect because async launches function: 1 first and it doesn't release control back to the event loop. You have no async functions in function: 1.

Node.js is a single-threaded asynchronous server. If you block the event loop with a long running CPU task then no other functions can be called until your long running CPU task finishes.

Instead for a big for loop, try making http requests. For example...

async = require('async')
request = require('request')

async.parallel([
    function(callback){
      request("http://google.jp", function(err, response, body) {
        if(err) { console.log(err); callback(true); return; }
        console.log("function: 1")
        callback(false);
      });
    },
    function(callback){
      request("http://google.com", function(err, response, body) {
        if(err) { console.log(err); callback(true); return; }
        console.log("function: 2")
        callback(false);
      });
    }
]);
Solution courtesy of: Daniel

Discussion

Javascrit is single-threaded unless you use special libraries/modules. So when you are executing this code it will execute the first function and then the second one.

The only thing that async.parallel does is execute all the functions and wait for all the responses, and then execute the code in the callback.

Because all the code you used is synchronous the result will be a synchronous.

Discussion courtesy of: Kaizo

This recipe can be found in it's original form on Stack Over Flow.