JavaScript, Node.js: is Array.forEach asynchronous?

Question

I have a question regarding the native Array.forEach implementation of JavaScript: Does it behave asynchronously? For example, if I call:

[many many elements].forEach(function () {lots of work to do}) 

Will this be non-blocking?

Answer 1

No, it is blocking. Have a look at the specification of the algorithm.

However a maybe easier to understand implementation is given on MDN:

if (!Array.prototype.forEach) {   Array.prototype.forEach = function(fun /*, thisp */)   {     "use strict";      if (this === void 0 || this === null)       throw new TypeError();      var t = Object(this);     var len = t.length >>> 0;     if (typeof fun !== "function")       throw new TypeError();      var thisp = arguments[1];     for (var i = 0; i < len; i++)     {       if (i in t)         fun.call(thisp, t[i], i, t);     }   }; } 

---

If you have to execute a lot of code for each element, you should consider to use a different approach:

function processArray(items, process) {     var todo = items.concat();      setTimeout(function() {         process(todo.shift());         if(todo.length > 0) {             setTimeout(arguments.callee, 25);         }     }, 25); } 

and then call it with:

processArray([many many elements], function () {lots of work to do}); 

This would be non-blocking then. The example is taken from High Performance JavaScript.

Another option might be web workers.

Answer 2

If you need an asynchronous-friendly version of Array.forEach and similar, they're available in the Node.js 'async' module: http://github.com/caolan/async ...as a bonus this module also works in the browser.

async.each(openFiles, saveFile, function(err){     // if any of the saves produced an error, err would equal that error });