Does a sparse Javascript array with whole number keys remain sorted by key?

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?

I haven't got the details to hand, but I found different behaviours in some browsers when I looked at this a couple of years ago... — Adam
– Adam, Commented May 2, 2015 at 19:37
If you initialize it as an object, with curly brackets, instead of square brackets (for arrays) you may get irregular behaviors between browsers. — Jonathan Gray
– Jonathan Gray, Commented May 2, 2015 at 19:40

Jonathan Gray · Accepted Answer · 2015-05-02 19:40:59Z

3

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.

edited May 2, 2015 at 19:40

answered May 2, 2015 at 19:35

Jonathan Gray

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3 Comments

Lightness Races in Orbit Over a year ago

Correct. If the OP iterated over the array properly (i.e. from 0 to a.length then he'd see that, if only because it's inherent). His loop ATM is a bit hacky.

pixelmike Over a year ago

I know undefined indices are created and I know that for..length would include them. What I don't know is if iterating it sparsely would always be in numeric order. That's the use case I'm interested in.

Jonathan Gray Over a year ago

@pixelmike Yes, it will be, as long as it is in fact an array object and not a regular old object.

Sushant · Accepted Answer · 2015-05-02 19:38:04Z

0

ECMA-262 does not specify enumeration order.

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

answered May 2, 2015 at 19:38

Sushant

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4 Comments

Jonathan Gray Over a year ago

That's initialized as a regular object, not an array.

Sushant Over a year ago

[] is an array constructor

Jonathan Gray Over a year ago

Correct. There is no array constructor used in the link you specified. An array is, by definition, an indexed (ordered) list.

dandavis Over a year ago

v8 keeps plain object properties in insertion order, expect for numerical indices, which it orders.

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