Is it worth caching the length of an array in a Javascript loop?

JavaScript performance comparison

Revision 56 of this test case created and last updated

Info

http://stackoverflow.com/questions/5349425/whats-the-best-to-loop-an-array-in-javascript

Preparation code

<script>
  var myArray = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20];
  var res = 0;
</script>

Test runner

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Testing in unknown unknown
Test Ops/sec
With caching
 
for (var i = 0, len = myArray.length; i < len; i++) {
  res += myArray[i];
}
window.res = res;
 pending…
Without caching
 
for (var i = 0; i < myArray.length; i++) {
  res += myArray[i];
}
window.res = res;
 pending…
Counting down
 
for (var i = myArray.length; i--;) {
  res += myArray[i];
}
window.res = res;
 pending…
caching length outside for loop
 
var len = myArray.length;

for (var i = 0; i < len; i++) {
  res += myArray[i];
}
window.res = res;
 pending…
while loop
 
var len = myArray.length;
while (!--len) {
  res += myArray[len];
}
window.res = res;
 pending…
for check index
 
for (var i = 0; myArray[i++];) {
  res += myArray[i];
}
window.res = res;
 pending…
upcount while
 
var len = myArray.length;
var ilen = len;
while (!--len) {
  res += myArray[ilen - len];
}
window.res = res;
 pending…
array shift
 
var len = myArray.length;
while (!--len) {
  res += myArray[0];
  myArray.shift();
}
window.res = res;
 pending…

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