具有整数键的稀疏Javascript数组是否仍按键排序？

Question

For example:

var a = [];
a[5] = 'five';
a[2] = 'two';
a[11] = 'eleven';
a[210] = 'two-ten';
a[4] = 'four';
a[56] = 'fifty-six';
a[1] = 'one';
a[39] = 'thirty-nine';

for( var i in a ) {
    if( a.hasOwnProperty(i) ) {
        console.log(i+":"+a[i]);
    }
}

Results in the following (for a few browsers I have handy to test.)

1:one
2:two
4:four
5:five
11:eleven
39:thirty-nine
56:fifty-six
210:two-ten

Is this standard, reliable behaviour?

Answer 1

They will always be in order, yes, but standards (as far as I'm aware) end up setting all of the in-between indexes as undefined.

Edit: If you initialize it as an object, with curly braces, instead of square braces (for arrays) you may get irregular behaviors between browsers.

Answer 2

ECMA-262 does not specify enumeration order.

Some browser implement them in ordered way others don't, like V8 to save memory.