How does JavaScript [] really work?

Question

I'm writing a JavaScript interpreter for extremely resource-constrained embedded devices (http://www.espruino.com), and every time I think I have implemented some bit of JavaScript correctly I realise I am wrong.

My question now is about []. How would you implement one of the most basic bits of JavaScript correctly?

I've looked through the JavaScript spec and maybe I haven't found the right bit, but I can't find a useful answer.

I had previously assumed that you effectively had two 'maps' - one for integers, and one for strings. And the array length was the value of the highest integer plus one. However this seems wrong, according to jsconsole on chrome:

var a = [];
a[5] = 42;
a["5"]; // 42
a.length; // 6

but also:

var a = [];
a["5"] = 42;
a[5]; // 42
a.length; // 6

So... great - everything is converted into a string, and the highest valued string that represents an integer is used (plus one) to get the length? Wrong.

var a = [];
a["05"] = 42;
a.length; // 0

"05" is a valid integer - even in Octal. So why does it not affect the length?

Do you have to convert the string to an integer, and then check that when converted back to a string, it matches?

Does anyone have a reference to the exact algorithm used to store and get items in an array or object? It seems like it should be very simple, but it looks like it actually isn't!

Answer 1

As the specs said, and was noted by others:

"A property name P (in the form of a String value) is an array index if and only if ToString(ToUint32(P)) is equal to P and ToUint32(P) is not equal to 2^32-1."

That's explain why in your scenario "5" is considered an array index and "05" is not:

console.log("5" === String("5" >>> 0));
// true, "5" is equal to "5", so it's an index

console.log("05" === String("05" >>> 0));
// false, "05" is not equal to "5", so it's not an index

Note: the Zero-fill right shift is the shortest way in JS to have a substitute of ToUint32, shifting a number by zero.

Answer 2

See MDN

It's possible to quote the JavaScript array indexes as well (e.g., years["2"] instead of years[2]), although it's not necessary. The 2 in years[2] eventually gets coerced into a string by the JavaScript engine, anyway, through an implicit toString conversion. It is for this reason that "2" and "02" would refer to two different slots on the years object and the following example logs true:

console.log(years["2"] != years["02"]);

So with a["5"] you are accessing the array while a["05"] sets a property on the array object.