I have a question about of the performance of using for loops (normal for, for each and for key in object) in JavaScript.

Just now I created this code to test:

var stopwatch = function(){
    var start;

    this.Start =function(){
        this.start = new Date().getTime();

    this.getDifference= function(){
        return new Date().getTime() - this.start;

var stopwatch = new stopwatch();
var length = 1000000;
var array = new Array(length);
for(var i = 0; i<length; i++){

for(var i = 0; i<length; i++){
var time = stopwatch.getDifference();
console.log("Time of normal for: "+time);
time = stopwatch.getDifference();
console.log("Time of for each: "+time);
for(key in array){
time = stopwatch.getDifference();
console.log("Time of for key in array: "+time);

The result is the following

Time of normal for: 29
Time of for each: 151
Time of for key in array: 803

It seems like the normal for is faster than the others implementations. Is it right?

  • 2
    \$\begingroup\$ You can check this link: jsperf.com/fastest-array-loops-in-javascript/24 \$\endgroup\$ Commented May 12, 2014 at 17:50
  • \$\begingroup\$ @MrMins thanks, that links was useful for me, so my code is ok? there is no other consideration to do with the code? And my result could be "real" and I could base my code practices in this little test? \$\endgroup\$
    – rahpuser
    Commented May 12, 2014 at 20:05

1 Answer 1


Well, here's some tips:

  • Never use for-in for arrays. for-in runs through the array properties.

    var a = [1,2,3]
    a.foo = 'bar'; // arrays are still objects
    for(var i in a){console.log(i)} // will run over foo
  • forEach though native, but AFAIK is still slow compared to while and for. Some say it suffers the same overhead of calling a function (activation records, stack, scope lookup etc.)

  • Consider async instead. By the looks of it, you want to loop through an array as fast as possible to avoid freezing the browser. You can go with WebWorkers, or Timers. Here's an example using setInterval. It's not as fast as loops, but you won't be seeing those "Unresponsive script" and "He's dead Jim!" warnings:

    var i = 1000000;
    var looper = setInterval(function(){
      /* do something */
      if(!--i) clearInterval(looper);
  • \$\begingroup\$ cool, that's why I was looking on faster implementations, I'm working with node.js and I trying to use the less webworkers possible. Thanks was a usuful answer. \$\endgroup\$
    – rahpuser
    Commented May 14, 2014 at 16:13

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