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Does the "for…in" loop in Javascript loop through the hashtables/elements in the order they are declared? Is there a browser which doesn't do it in order?
The object I wish to use will be declared once and will never be modified.

Suppose I have:

var myObject = { A: "Hello", B: "World" };

And I further use them in:

for (var item in myObject) alert(item + " : " + myObject[item]);

Can I expect 'A : "Hello"' to always come before 'B : "World"' in most decent browsers?

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Becasue they would only be testing a subset of potential browsers and variants. Not to mention any future browsers. It is plain wrong to assume a non-failing test provides any sort of concrete proof. –  James Hughes Nov 11 '08 at 12:23
I doubt my own limited javascript ability will be better than the SO crowd. Besides who knows what strange browser lurks out there? And you can see in the answer that GChrome does have a bug which will not be apparent in my simple example case. –  chakrit Nov 11 '08 at 13:31
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7 Answers

up vote 132 down vote accepted

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 = {
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.

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Not really. Chrome doesn't implement the same order as other browsers: code.google.com/p/v8/issues/detail?id=164 –  Tim Down Nov 30 '10 at 23:13
@Tim, Thanks for the nudge. I've updated the answer to reflect the current state of reality. –  Borgar Dec 3 '10 at 0:02
"Use an array if order is important to you": What about when you're using JSON? –  HM2K Feb 10 '11 at 23:40
@HM2K, Same thing, spec says "An object is an unordered collection of zero or more name/value pairs." JSON isn't JavaScript: A server doesn't need to (and probably won't) respect the order you hand it. –  Borgar Feb 12 '11 at 1:40
Firefox since its 21 version doesn't seem to respect the insertion order anymore. –  PomCompot Jun 14 '13 at 7:26
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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(':'));
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);


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"]


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Verified and added version, IE9 and Node 0.6 –  some Aug 27 '12 at 16:45
what's the problem here? Exactly as the specification says, it's an unordered list of key/val pairs. –  nickf Feb 12 '13 at 9:28
nickf: the implementations are right in that they do what the spec says. I think everyone agrees that the javascript spec is.. well, I don't want to use the word "wrong", how about "extremely stupid and annoying"? :P –  boxed Feb 25 '13 at 2:05
@boxed: Think about objects as a hash map (table/whatever). In most languages (Java, Python, etc) these kinds of data structures are not sorted. So it is not surprising that this is the same case in JavaScript as well and it certainly does not make the specification wrong or stupid. –  Felix Kling Apr 12 '13 at 9:53
I honestly don't see the point of this answer (sorry). The order in which properties are iterated over is an implementation detail, and browsers use different JavaScript engines, so it is expected that the order differs. This won't change. –  Felix Kling Apr 12 '13 at 9:59
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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.

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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.

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which is why he's asking the question, instead of just assuming :p –  Jamie Pate Apr 21 '13 at 21:48
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in IE6, the order is not guaranteed.

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The order cannot be trusted. Both Opera and Chrome return the list of properties unordered.

<script type="text/javascript">
var username = {"14719":"A","648":"B","15185":"C"};

for (var i in username) {
  window.alert(i + ' => ' + username[i]);

The code above shows B, A, C in Opera and C, A, B in Chrome.

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Iteration order is also confused with respect to deleting of properties, but in this case with IE only.

var obj = {};
obj.a = 'a';
obj.b = 'b';
obj.c = 'c';

// IE allows the value to be deleted...
delete obj.b;

// ...but remembers the old position if it is added back later
obj.b = 'bb';
for (var p in obj) {
    alert(obj[p]); // in IE, will be a, bb, then c;
                   // not a, c, then bb as for FF/Chrome/Opera/Safari

The desire for changing the spec to fix the iteration order seems to be quite a popular desire among developers if the discussion at http://code.google.com/p/v8/issues/detail?id=164 is any indication.

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Thanks for pointing this out! –  chakrit May 3 '13 at 7:05
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