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

JavaScript performance comparison

Revision 74 of this test case created by bradleyg

Info

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

Preparation code

 
<script>
  Benchmark.prototype.setup = function() {
      var myArray = [];
      for (var i = 1000; i--;) myArray.push(i);
  };
</script>

Test runner

Warning! For accurate results, please disable Firebug before running the tests. (Why?)

Java applet disabled.

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

for (var i = 0; i < len; i++) {
  myArray[i];
}
 pending…
while loop
 
var len = myArray.length;
while (len--) {
  // blah blah
  myArray[len];
}
 pending…
for check index
 
for (var i = 0; myArray[i++];) {
  myArray[i];
}
 pending…
caching, !==
 
for (var i = 0, len = myArray.length; i !== len; i++) {
  myArray[i];
}
 pending…
destructive while
 
var a;
while ((a = myArray.shift()) !== undefined) {
  a;
}
 pending…
nondestructive while
 
var a, i = 0;
while ((a = myArray[i++]) !== undefined) {
  a;
}
 pending…
forEach
 
myArray.forEach(function(key){
  myArray[key]
});
 pending…

Compare results of other browsers

Revisions

You can edit these tests or add even more tests to this page by appending /edit to the URL. Here’s a list of current revisions for this page:

0 comments

Add a comment