Javascript associative array and iteration order

I just noticed that some browsers, like IE9 and Chrome, don't preserve insertion order of keys when iterating with a for-in loop ["for (key in array)"]. Although technically Javascript never guaranteed preserving order in a for-in loop, so many popular browsers did anyway that I was taken by surprise.

The Chrome behavior is especially odd - it preserves order of insertion for non-numeric keys, but iterates numeric keys in numeric order first. It also coerces numeric strings to numbers.

This code example illustrates

var foo=new Object();



foo["111_"] = 1;

foo["222_"] = 2;

foo["333_"] = 3;

foo["444"] = 4;

foo["555"] = 5;

foo["666"] = 6;



var str = "";

for (var key in foo) { 

  str = str + foo[key] + " ";

}

document.write(str);

In Chrome this prints "4 5 6 1 2 3". In IE 7, IE 8, and Firefox, it prints "1 2 3 4 5 6".

This behavior (particularly the coercion of numeric strings) has been tracked as a bug in Chrome - but unclear whether it will ever be fixed
http://code.google.com/p/chromium/issues/detail?id=37404

ECMAScript spec says "order of enumerating... is not defined"
http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-262.pdf

Written on August 24, 2011