Using a for..in loop on an array can cause problems if you (or others) extended Array.prototype, looping through the numeric indices is much safer!

$.each is not intended to be used on huge empty arrays, you can easily write a function based on $.each that skips unnecessary elements :)

The native forEach method (if implemented) doesn't skip undefined properties either.

I just recently experienced this problem of properties added to the prototype popping up amongst array elements when the array is traversed the wrong way (with a for/in loop).

Filtering out the object properties to keep only array elements can be achieved as follows, I think:

for (ind in array) {

if (!isNaN(ind)) continue;

//array element processing...

}

Is this not incredibly more efficient than filtering out possibly hundreds or thousands of non-existent numeric indices?

The reason for all these unused array indices is that the data comes from a database and the unique record number is used as key value when it is stored in arrays. I may have just a few elements with a very high key value stored in an array.

Any advice on other / better ways of doing things is welcome.

In the meanwhile, I think I will write a variant of the $.each iterator to more efficiently process arrays.

Correction: the above code should read "if(isNaN(ind)) continue"...

• isNaN won't help much as all the indices "exist" less then the length property (more precisely they are undefined)

• for objects you could use the hasOwnProperty method

• but here this is unnecessary, just use a loop like this:

for(var i=0, l=array.length; i<l; i++){ 
  if(typeof array[i]=="undefined") continue;
  ...
}

• function calls also take some (very little) time, with a simple loop there isn't any

And since this is just a bug tracker such topics are better suited on the mailing list: http://groups.google.com/group/jquery-en/

The for/in is not a safe way to deal with an Array because of the issues that balazs.endre mentions. jQuery.each is also used for array-like objects such as the arguments list of a function. Using for/in would break that.