Why is JavaScript's For…In loop not recommende

Question

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• Why is using “for…in” with array iteration a bad idea? 26 answers

I read somewhere (sorry, I can't find the link) that the For...In loop is not recommended for arrays. It is said here: http://www.openjs.com/articles/for_loop.php that it is meant for associative arrays, and in http://www.w3schools.com/js/js_loop_for_in.asp that is for iterating through all the properties of an object (It does not say that it can be used on arrays). I do not know who to believe. I don't want this question to become a debate. I just want to know if I could use this in my code without unforeseen side effects. Thanks!

Answer 1

Iterating over an array using for(... in ...) will not get you numeric keys, it gets you string values.

It will get you properties defined on the prototype, so if any code extends Array, e.g. by doing:

Array.prototype.each = ...;

then you will see the property each.

There is no gaurantee that you will get the properties in array index order. E.g. try iterating over

var arr = []
arr[1] = 1;
arr[0] = 0;

On a lot of browsers you will get 1 before 0.

And you are not guaranteed to get all indices. Try iterating over

[0,,,3]

you will not get the index 1 or 2.

Answer 2

I've tested array iteration in several browsers (FireFox 3, Opera 9, IE6, IE9 beta, Chrome) and it works fine; I seem to recall some cross-browser incompatibility but I must be mistaken.

There is still a caveat though:

As you've mentioned, the for ... in syntax is used for iterating over the properties of an object; so, with an array, all the properties of that array object will also be iterated over along with the elements. Normally, the array object only has properties corresponding to the keys in it, but if another script modifies Array.prototype (e.g. some framework), then the added methods/attributes would unexpectedly show up in the iteration as well.

Answer 3

An array is an object, and array elements are simply properties with the numeric index converted to string. For example, arr[123] refers to a property "123" in the array object arr.

The for ... in construct works on all objects, not just arrays, and that is cause for confusion.

When somebody for ... in an array, most often the programmer intends to iterate just all the elements, even most likely in order. For example, if the array holds a bunch of numbers, then the programmer most likely intends to iterate a stream of numbers. The semantics is so similar to array iteration in other programming languages that it is very easy to get confused.

In JavaScript, this construct does not iterate array elements in order. It iterates all the array's property names (including the names of inherited prototype functions, any properties added to it, any other non-element properties added to it etc.), and not in order at all. In earlier browsers, it will even find the property length, although in recent browsers these properties are now defined to be hidden for this exact reason -- people keep tripping on it!

With the array of integers above, you get not a stream of numbers, but a stream of text strings. And not the element values, but the names of the properties (which are just the numeric indices not in any order). This is most likely not what the programmer means, if he/she comes from another programming language. If the elements stored in the array happen to be similar numeric values, it confuses the hell out of everybody.

That's why you shouldn't do it. You shouldn't use a language construct that looks like it does obvious things, but actually does things that are completely different. It creates bugs that are very obscure and very difficult to find.

Answer 4

A couple of good reasons are given in the Prototype.js library documentation: http://www.prototypejs.org/api/array

Basically, using for...in to iterate an array is brittle since any other code can add properties to the Array prototype, which will then become enumerable properties on every array object.