Why is 'for(var item in list)' with arrays considered bad practice in JavaScript?

Question

Given a simple zero based, numerically indexed array:

var list = [\'Foo\', \'Bar\', \'Baz\'];

Many times, I have noticed that when someone

Answer 1

If you use for/in like that, item enumerates through string values "0", "1", ..., so not the actual objects in the list. So the the 'item' in the first snippet is more like the i in the second snippet,not the item. Furthermore string values are enumerated where you'd expect numbers. And you get in trouble when you properties to the list, like array.ID = "a123", as they will get enumerated also.

But with these downsides, I still think the syntax is very useful, if your team is aware of what it does.