What's the fastest way to iterate over an object's properties in Javascript?

Question

I know that I can iterate over an object's properties like this:

for (property in object)
{
    // do stuff
}

I also know that the fastest way to iterate over an array in Javascript is to use a decreasing while loop:

var i = myArray.length;
while (i--)
{
    // do stuff fast
}

I'm wondering if there is something similar to a decreasing while loop for iterating over an object's properties.

Edit: just a word about the answers concerned with enumerability - I'm not.

Answer 1

An object's properties are unordered by definition. The lack of order means that there is no "forwards", and therefore there is no "backwards" either.

Answer 2

The for/in loop is the best way to enumerate properties of a Javascript object. It should be understood that this will only loop through "enumerable" properties, and in no particular order. Not all properties are enumerable. All properties/methods added programmatically via your own Javascript code will be enumerable, but predefined properties/methods that are inherited (such as toString) are not usually enumerable.

You can check enumerability like so...

var o = new Object();
alert(o.propertyIsEnumerable("toString"));

Answer 3

If you don't know the names of the properties, for..in is a good way to enumerate them. If you do, you're better off with explicit dereference.

Answer 4

1) There's no other way to enumerate properties than using for..in (that is until ES5 Object.keys is implemented). Perhaps if you stated your original problem, someone could suggest a way to optimize.

2) I find it hard to believe that the actual enumeration is taking more than whatever you do with the properties in the loop body.

3) You didn't specify what platform you're developing for. The answer would probably depend on it, and the available language features depend on it too. E.g. in SpiderMonkey (Firefox JS interpreter) you could use for each(var x in arr) (docs) if you actually need the values, not the keys. It's faster than for (var i in arr) { var x = arr[i]; ... }.

4) You probably just didn't include it in your snippet, but a faster way to do a for..in iteration is to make sure the variables you use in the loop are declared in the function with the loop, i.e.:

//slower
for (property in object) { /* do stuff */ }

//faster
for (var property in object) { /* do stuff */ }

5) Related to (4): while trying to optimize a Firefox extension I once noticed that extracting a tight loop into a separate function improved its performance (link). (Obviously, it doesn't mean you should always do that!)

Answer 5

Explicit use of Iterator in JavaScript 1.7+ might be faster or slower. Of course this will only iterate an object's own properties. The catch statement also might be faster with ex instanceof StopIteration replaced with ex === StopIteration.

var obj = {a:1,b:2,c:3,d:4,e:5,f:6},
   iter = new Iterator(obj, true);

while (true) {
    try {
     doSomethingWithProperty(iter.next());
    } catch (ex if (ex instanceof StopIteration)) {
     break;
    }
}