Fastest array loops in Javascript

JavaScript performance comparison

Revision 171 of this test case created

Info

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

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

  function someFn(ix) {
    return ix * 5 + 1 / 3 * 8;
  }
</script>

Test runner

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Testing in unknown unknown
Test Ops/sec
For loop, basic
for (var i = 0; i < arr.length; i++) {
  someFn(arr[i]);
}
pending…
While loop, basic
var i = 0;
while (i < arr.length) {
  someFn(arr[i]);
  i++;
}
pending…
For loop, cached
for (var i = 0, len = arr.length; i < len; i++) {
  someFn(arr[i]);
}
pending…
For loop, i--
for (var i = arr.length; i > 0; i--) {
  someFn(arr[i]);
}
pending…
Do-while loop, i--
var i = arr.length - 1;
do {
  someFn(arr[i]);
}
while (i--);
pending…
Do-while loop, --i
var i = arr.length;
if (i > 0) {
  do {
    someFn(arr[i]);
  }
  while (--i);
}
pending…
For..in loop
for (var i in arr) {
  someFn(arr[i]);
}
pending…
Order
var i;
while (i = arr.shift()) {
  someFn(i);
}
pending…
forEach
arr.forEach(function(v) {
  someFn(v)
})
pending…
order validation
var i;
while ((i = arr.shift()) !== undefined) {
  someFn(i);
}
pending…
While ! Undefined
var a, i = 0;
while ((a = arr[i++]) !== undefined) {
  someFn(a);
}
pending…
Pop
var i;
while (i = arr.pop()) {
  someFn(i);
}
pending…
Order w/ array copy
var i, copy = arr.slice(0);
while (i = copy.shift()) {
  someFn(i);
}
pending…
copy reverse pop
var i, arr2 = arr.slice().reverse();
while (i = arr2.pop()) {
  someFn(i);
}
pending…
While ! Undefined (array end + &&)
var a, i = 0,
  len = arr.length;
while (i !== len) {
  ((a = arr[i++]) !== undefined) && someFn(a);
}
pending…
While ! Undefined (array end)
var i = 0,
  len = arr.length;
while (i !== len) {
  someFn(arr[i++]);
}
pending…
Generated + eval
var i = 0,
  len = arr.length,
  body = "",
  f = someFn,
  a = arr;
while (i !== len) {
  body += "f(a[" + i + "]);";
}
eval(body);
pending…
While reverse
var i = arr.length;
while (i--) {
  someFn(arr[i]);
}
pending…
For loop, cached #2
var i = 0,
  l = arr.length;
for (; i < l; i++) {
  someFn(arr[i]);
}
pending…
for loop, inversed cached
var l = arr.length;
for (; l !== 0;) {
  someFn(arr[--l]);
}
pending…
while
var l = arr.length;
while (l--) {
someFn(arr[l]);
}
pending…

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Revisions

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