for..in loop over an Array iterates also over the prototype functions

Question

I've got a serious bug, which I've never seen before. First of all I've a simple Array:

var myArray = ["123", "456", "789"]

Now I want to iterate over this Array with a for..in - loop:

function mapData(list) {
for ( var i in list) {
    var item = list[i];
    if (item) {
      // do something
    }
  }
}

After calling the method with mapData(myArray), firebug shows in the debugger this:

Loop: i = 0; item = 123;

Loop: i = 1; item = 456;

Loop: i = 2; item = 789;

Loop: i = compare;

Loop: i = union;

Loop: i = remove;

Loop: i = select;

Loop: i = contains;

So I think that are the prototype functions. But why? Any Ideas?

As I mentioned, I've never seen this before...

for..in is supposed to do this; it's for iterating Objects, not Arrays, so if there are any enumerable properties, inherited or otherwise, they will be iterated over. — Paul S.
– Paul S., Commented Sep 6, 2013 at 13:48

Explosion Pills · Accepted Answer · 2013-09-06 13:51:14Z

4

Do not use for..in to iterate over Array. This will iterate over all enumerable properties of the Array object and may not iterate over them in order. There are two alternatives:

Array.forEach (not supported by IE8-, although there is probably a shim).
A simple for loop

answered Sep 6, 2013 at 13:51

Explosion Pills

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5 Comments

Tim Down Over a year ago

Nitpick: forEach is a method of Array.prototype (and hence all arrays) rather than Array.

Paul S. Over a year ago

It won't iterate over all properties; only enumerable ones.

Explosion Pills Over a year ago

@TimDown I was considering writing Array.prototype.forEach but I thought it would be confusing :(

Paul S. Over a year ago

@ExplosionPills I edited the MDN article's title so the link shows the right thing now anyway

Tim Down Over a year ago

Yeah, I'm not a model of consistency on that either.

Denys Séguret · Accepted Answer · 2019-09-18 06:58:36Z

1

That's not a bug, you're just iterating over all enumerable array properties, and a library you use added functions to Array.prototype like this :

Array.prototype.union = function(...)

The library can be fixed by making the functions not enumerable by setting them using defineProperty :

Object.defineProperty(Array.prototype, 'union', {value:function(){...}});

But you should never use for..in to iterate over an array.

Iterate like this :

for (var i=0; i<list.length; i++) {

or like this:

for (let item of list) {

edited Sep 18, 2019 at 6:58

answered Sep 6, 2013 at 13:47

Denys Séguret

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