Question

JSLint keeps complaining about things like this

var myArray = [1, 2, 3];
for (var value in myArray)
{
   // BLAH
}

Saying that I should wrap it in an if statement. I realize you need to wrap it if you are looping over an object’s properties, but here what should I put in the if statement to do the correct filtering.

Additionally when I do something like

for (var i = 0; i < 10; i++)
{
   // foo
}

for (var i =0; i < 20; i++)
{
   // bar
}

It complains that i has already been defined. How do I prevent this other than using different variable names?

Answer 1

JSLint whinges about a lot that’s not really harmful. In this case it’s right to complain about for...in, because that’s the wrong construct to loop over an Array.

This is because you will get not only the numeric keys, but also any other arbitrary properties that have been added to the array or its Array.prototype. The latter typically comes from extension utility functions added by frameworks.

Whilst you can defeat that case with hasOwnProperty to check it’s not a prototype member, it’s uglier than just doing it the proper way with for (var i= 0...) so why bother.

Also, with for...in you won’t necessarily get the items in numerical order as you might expect.

It complains that i has already been defined. How do I prevent this other than using different variable names?

Yeah, you can ignore that one.

It wants you to remove the var from the second for (i..., because declaring a variable twice in the same scope doesn’t do anything. However I would recommend leaving the var there because it doesn’t do any harm, and if you move the loop to another block you don’t want it to be suddenly scribbling on globals.