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Editorial Team
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Asked: 2026-05-31T03:12:20+00:00

Are there any performance difference between var a = [10,20,30,40];// Assume we have thousands

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Are there any performance difference between 

var a = [10,20,30,40];// Assume we have thousands of values here

// Approach 1
var i, len = a.length;
for(i=0;i<len;i++){
  alert(i);
  alert(a[i]);
}

// Approach 2   
for( i in a ){
  alert(i);
  alert(a[i]);
}
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1 Answer

  1. Editorial Team

    Use for (var i = 0, len = a.length; i < len; i++) because it’s way faster and it’s the correct way or iterating the items in an array.

    First: It’s not correct to iterate arrays with for (i in a) because that iteration will include enumerable properties in addition to array elements. If any methods or properties have been added to the array, they will be part of the iteration when using for (i in a) which is never what you want when trying to traverse the elements of the array.

    Second: The correct option is a lot faster (9-20x faster). See this jsPerf test which shows the for (var i = 0; i < len; i++) option to be about 9x faster in Chrome and even more of a speed difference in Firefox: http://jsperf.com/for-loop-comparison2.

    enter image description here

    As an example of the problems that can occur when using for (var i in a), when I use that when the mootools library is included in the project, I get all these values for i:

    0
1
2
3
$family
$constructor
each
clone
clean
invoke
associate
link
contains
append
getLast
getRandom
include
combine
erase
empty
flatten
pick
hexToRgb
rgbToHex

    which appears to be a bunch of methods that mootools has added to the array object.

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