Do javascript arrays and objects have a set order?

Question

This question is vital to one of my current projects involving creating HTML tables from JSON objects. I was thinking I could create functions that would sort my Arrays/Objects before rendering them as HTML. My only concern is that order doesn't matter in JavaScript, and if I were to create a new Array with the same data (in a different order) as another Array they would end up identical.

I can't think of a fast way to test this, so I'm asking here.

Answer 1

Others have answered on arrays, so I just wanted to provide some info on objects.

On that, the standard is non-existent, but it's de facto definition is that an object will enumerate in insertion order, with the exception that numbers are placed in ascending order and enumerated first. I say typically, but this behavior is nowhere near standardized (nor do frameworks like jQuery standardize it, AFAIK).

You can test browsers using this jsFiddle:

http://jsfiddle.net/7cCpu/4/

The object {"foo":"bar", "bar":"foo", "baz":"baz", "3":3, "2":2, "1":1} enumerates as follows:

foo, bar, baz, 3, 2, 1 // insertion order

1, 2, 3, foo, bar, baz // Chrome enumeration
1, 2, 3, foo, bar, baz // Opera
1, 2, 3, foo, bar, baz // IE9
foo, bar, baz, 3, 2, 1 // Firefox (!!!)

I don't have Safari installed, but I assume it's the same as Chrome. In any case, the point is that you can make assumptions -- it's not random -- but it's probably a better idea to use an array if you depend on exact enumeration.

Even nastier is what duri pointed out above, where deleting and replacing the value for a key alters things further. Watch what happens when I delete bar and do Object.bar = "foo" then enumerate:

1, 2, 3, foo, baz, bar // Chrome enumeration
1, 2, 3, foo, baz, bar // Opera
1, 2, 3, foo, bar, baz // IE9 (!!!)
foo, baz, 3, 2, 1, bar // Firefox (!!!)

Answer 2

Arrays are enumerated by (numeric) index.

Objects aren't - the order isn't defined, indeed the Javascript standards explicity state that the order is undefined. For more detail see Does JavaScript Guarantee Object Property Order?

Answer 3

Good example when order of object properties differs is this code:

var o = { foo: 1, bar: 2, baz: 3 };
delete o.bar;
o.bar = 2;
for (var i in o)    {
    document.write(o[i]);
}

This displays 132 in Firefox, but 123 in Internet Explorer.