caching length

JavaScript performance comparison

Revision 95 of this test case created

Preparation code

 
<script>
Benchmark.prototype.setup = function() {
    var arr = [];
    for (var i = 0; i < 1000; i++) {
      arr[i] = Math.random();
    }
};
</script>

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Testing in unknown unknown
Test Ops/sec
cached
for (var i = 0, len = arr.length; i < len; i++) {
  arr[i] *= 2;
}
pending…
not cached
for (var i = 0; i < arr.length; i++) {
  arr[i] *= 2;
}
pending…
u wot m8
var len = 2;
len *= arr.length;
len -= len / 2;
for (var i = 0; i < len; i++) {
  arr[i] *= 2;
}
pending…
while loop
var len = arr.length;
while (len--) {
  arr[len] *= 2;
}
pending…
do-while loop
var len = arr.length;
do {
  arr[len] *= 2;
} while (len--);
pending…
Prefix Increment
for (var i = 0; i < arr.length; ++i) {
  arr[i] *= 2;
}
pending…
Prefix + Cached
for (var i = 0, len = arr.length; i < len; ++i) {
  arr[i] *= 2;
}
pending…
Array map
arr.map(function(el) {
  return el * 2;
});
pending…
Unrolled
var iterations = Math.floor(arr.length / 8);
var leftover = arr.length % 8;
var i = 0;

if (leftover > 0) {
  do {
    arr[i] *= 2;
  } while (--leftover > 0);
}

do {
  arr[i++] *= 2;
  arr[i++] *= 2;
  arr[i++] *= 2;
  arr[i++] *= 2;
  arr[i++] *= 2;
  arr[i++] *= 2;
  arr[i++] *= 2;
  arr[i++] *= 2;
} while (--iterations > 0)
pending…

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