Is the order of fields in a javascript object predictable when looping through them?

Question

In php, if you have the following code:

$map = array(
 "first" => 1,
 "second" => 2
);
$map["third"] = 3;
foreach($map as $key => $value) {
 // code
}

You know the entries will be listed in the order they have been added to the array.

Now, can I assume the same rule applies to the Javascript equivalent below?

map = {
 "first": 1,
 "second": 2
};
map["third"] = 3;
for (key in map) {
 // code
}

---

This is a duplicate of: Elements order - for (... in ...) loop in javascript

• Borgar: possible duplicate of Elements order - for (... in ...) loop in javascript

Answer 1

Most browsers will loop through the properties in the order they were added to the object, but the Javascript standard says the order is undefined -- so you shouldn't rely on this behavior. For example, I read a blog post a while back about how Google Chrome didn't always exhibit this behavior.

If you need the ordered functionality, you should create a new class for yourself that can use both object or numeric keys.

• Julian Aubourg: Thanks a lot for the link. What John says is that, though it is not standard, browser vendors apply it as if it was (so the Chrome team will fix the "bug").
• chroder: Yup. Though I still don't think you should rely on the behavior. Safer to just create a custom class where you can define the behavior yourself.

Answer 2

No, the behavior depends on implementation and it is not guaranteed. Use an array when the order needs to be preserved.

• Julian Aubourg: Yes but (in the standard): "The mechanics of enumerating the properties (step 5 in the first algorithm, step 6 in the second) is implementation dependent. The order of enumeration is defined by the object." So enumeration itself is implemtation dependent, but the order is object dependent, no?
• Christoph: @Julian: the next edition 3.1 of ECMA-262 is less ambiguous: "The mechanics and order of enumerating the properties [...] is not specified."