Loop Through Array Without Wasteful Lookups

var arLen=myArray.length;

for ( var i=0, len=arLen; i<len; i++ ){
  // do something with myArray[i]
}

5 Responses

1. Alex says:

   12/02/2009 at 8:29 am

   While you are correct that it speeds things up, it’s not because the alternative has to count the length each time. With a language like java where the length/size/count method on the array object is in fact a method (that presumably does some calculation to figure out the size) then it is better to cache the size in the way you described.

   The reason that it is slightly faster this way, is because the browser does have to dereference the array object each time it gets its length, not recalculate the length.

   Also if you want to make another very slight improvement here you can do:
   
for (var i = 0, len=myArray.length; i < len; ++i) {
// loop-the-loop
}

   where the key here is the ++i instead of i++.

   Reply
   • LuK says:

     12/21/2009 at 6:52 am

     Why is this faster?

     Reply
     • Matti says:

       12/29/2009 at 6:54 am

       The post-increment operator (i++) first returns the current value of i and then increments it by one. The pre-increment operator (++i) first increments the value and then returns the value.

       As you can see, the compiler needs to hold the previous state of i as return value for i++ whereas for ++i it returns the incremented state. The difference is really minuscule though but hey, we just saved an otherwise wasted lookup. ;-)
2. James says:

   02/22/2010 at 2:30 pm

   Another tiny trick that may help speed things up is to count down to zero if possible, as it is quicker to do a compare to zero, than to do a ‘less than’ (which is fundamentally a subtraction followed by a comparison with zero.

   Reply
3. Sumeet Chawla says:

   05/08/2010 at 12:46 pm

   One more thing I would like to suggest is that why have you used another variable len to store the value of the length of the array:

   len=arLen

   why not just:
   
var arLen=myArray.length;

   for ( var i=0; i<arLen; i++ ){
// do something with myArray[i]
}


   Very interesting points for a simple loop structure :) Really very conceptual.. Love the comments!

   Reply