Elements order in a "for (… in …)" loop

From the ECMAScript Language Specification, section 12.6.4 (on the for .. in loop):

The mechanics of enumerating the properties is implementation dependent. The order of enumeration is defined by the object.

And section 4.3.3 (definition of "Object"):

It is an unordered collection of properties each of which contains a primitive value, object, or function. A function stored in a property of an object is called a method.

I guess that means you cant rely on the properties being enumerated in a consistent order across JavaScript implementations. (It would be bad style anyway to rely on implementation-specific details of a language.)

If you want your order defined, you will need to implement something that defines it, like an array of keys that you sort before accessing the object with it.

The elements of an object that for/in enumerates are the properties that don't have the DontEnum flag set. The ECMAScript, aka Javascript, standard explicitly says that "An Object is an unordered collection of properties" (see http://www.mozilla.org/js/language/E262-3.pdf section 8.6).

It's not going to be standards conformant (i.e. safe) to assume all Javascript implementations will enumerate in declaration order.

Quoting John Resig:

Currently all major browsers loop over the properties of an object in the order in which they were defined. Chrome does this as well, except for a couple cases. [...] This behavior is explicitly left undefined by the ECMAScript specification. In ECMA-262, section 12.6.4:

The mechanics of enumerating the properties ... is implementation dependent.

However, specification is quite different from implementation. All modern implementations of ECMAScript iterate through object properties in the order in which they were defined. Because of this the Chrome team has deemed this to be a bug and will be fixing it.

All browsers respect definition order with the exception of Chrome and Opera which do for every non-numerical property name. In these two browsers the properties are pulled in-order ahead of the first non-numerical property (this is has to do with how they implement arrays). The order is the same for Object.keys as well.

This example should make it clear what happens:

var obj = {
  "first":"first",
  "2":"2",
  "34":"34",
  "1":"1",
  "second":"second"
};
for (var i in obj) { console.log(i); };
// Order listed:
// "1"
// "2"
// "34"
// "first"
// "second"

The technicalities of this are less important than the fact that this may change at any time. Do not rely on things staying this way.

In short: Use an array if order is important to you.

Bumping this a year later...

It is 2012 and the major browsers still differ:

function lineate(obj){
    var arr = [], i;
    for (i in obj) arr.push([i,obj[i]].join(':'));
    console.log(arr);
}
var obj = { a:1, b:2, c:3, "123":'xyz' };
/* log1 */  lineate(obj);
obj.a = 4;
/* log2 */  lineate(obj);
delete obj.a;
obj.a = 4;
/* log3 */  lineate(obj);

gist or test in current browser

Safari 5, Firefox 14

["a:1", "b:2", "c:3", "123:xyz"]
["a:4", "b:2", "c:3", "123:xyz"]
["b:2", "c:3", "123:xyz", "a:4"]

Chrome 21, Opera 12, Node 0.6, Firefox 27

["123:xyz", "a:1", "b:2", "c:3"]
["123:xyz", "a:4", "b:2", "c:3"]
["123:xyz", "b:2", "c:3", "a:4"]

IE9

[123:xyz,a:1,b:2,c:3] 
[123:xyz,a:4,b:2,c:3] 
[123:xyz,a:4,b:2,c:3]