Question

Possible Duplicate:
JavaScript for…in vs for
difference between for..in and for loops, and counter declaration

Is there any particular reason that for loops in arrays should be coded like for (var i = 0; i < foo.length; i++) { whereas loops in objects are just for (var i in foo) { Is it because of how objects are set up vs how arrays are set up? (I know arrays are a type of object btw) or just another nitpicky programming convention.

Answer 1

Well, the main reason is because for...in only iterate over defined members.

So,

var arr = [];
arr[10] = "hi";
for(var x in arr)
{
    alert(x);
}

will only show the one value that is defined. Sometimes this is a good thing– usually not.

Also, for...in loops over properties, not array indices (in javascript an array is just an a regular object with properties whose names are the values you assign to the array)

So when you use for..in you can get weird results, like:

1

2

3

length

toString

etc.

Using the typical for syntax is just the better option for arrays. With objects, the for...in makes more sense, since the undefined issues don’t come up as often. Either way, I like to use the for(var x; x < y; x++) syntax, though. It just feels better in js.