Question

I can add an item to an array it and I can access that item, but the length reports 0. Why?

var arr = [];
arr[4294967300] = "My item";
console.log(arr[4294967300], arr.length); // Outputs "My item", 0

Answer 1

That’s because the index is so big that it gets turned into a property instead, hence the length is 0.

According to the ECMAScript documentation, a particular value p can only be an array index if and only if:

(p >>> 0 === p) && (p >>> 0 !== Math.pow(2, 32) - 1)

Where >>> 0 is equivalent to ToUint32(). In your case:

4294967300 >>> 0 // 4

By definition, the length property is always one more than the numeric value of the biggest valid index; negative indices would give you the same behaviour, e.g.

arr[-1] = 'hello world';
arr.length; // 0
arr['-1']; // 'hello world'

If your numbers range between what’s valid (and gets used as an index) and “invalid” (where it gets turned into a property), it would be better to cast all your indices to a string and work with properties all the way (start with {} instead of Array).