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

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.