for loop i++ vs in...

JavaScript performance comparison

Revision 67 of this test case created by for loop i++ vs in...


Testing the best overall performers (across both desktop and mobile) from these jsperf tests:

Caching array length

Different kinds of loop

Interestingly, it would seem that current 2012 browsers have all optimized their Javascript engines for basic for and while loop syntax. This means we can probably abandon all the 'trick' loops (that used to run faster) and just go with whatever loop syntax we find the most readable.

Preparation code

  // Populate the base array
  var arr = [];
  for (var i = 0; i < 1000; i++) {
    arr[i] = 'value' + i;

  function someFn(ix) {
    return ix * 5 + 1 / 3 * 8;

Test runner

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Testing in unknown unknown
Test Ops/sec
For loop, basic
for (var i = 0; i < arr.length; i++) {
for .. in
for ( var i in arr ) {

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