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
Asked
Active
Viewed 133 times
4
Stuart Wakefield
- 6,294
- 24
- 35
-
I think JavaScript is being stretched to limit. Oh JavaScript! – codingbiz Oct 16 '12 at 10:29
-
well this kind of index would stretch any programming language – Michal Oct 16 '12 at 10:31
-
1just over the limit.. coincidence? Maybe not ;) – st3inn Oct 16 '12 at 10:31
-
if you would like to get the "length" of that object see http://stackoverflow.com/questions/5223/length-of-javascript-object-ie-associative-array – Michal Oct 16 '12 at 10:34
-
Actually in this example using that code would return `1`, but if I was expecting it to act like an array I would expect to see a length of `4294967301` – Stuart Wakefield Oct 16 '12 at 11:15
1 Answers
6
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).
Ja͢ck
- 170,779
- 38
- 263
- 309
-
Honest question, is there any reason you used a zero-fill bit shift over the other shifts or a bitwise or `p | 0`? – Stuart Wakefield Oct 16 '12 at 11:07
-
@StuartWakefield it's a special shr operation in the sense that it also casts to an unsigned 32 bit integer, hence the triple bracket for extra kick :) – Ja͢ck Oct 16 '12 at 11:42
-
Don't all bitwise operators cast operands to 32 bit integers? – Stuart Wakefield Oct 16 '12 at 11:43
-
1@StuartWakefield Yes, all bit wise operators cast to int32, but >>> clears the sign bit as well, turning it into uint32 – Ja͢ck Oct 16 '12 at 12:26
-
Nice one, that answers it perfectly `-1 | 0` gives `-1` but `-1 >>> 0` gives `4294967295`. Cheers for finding that out for me – Stuart Wakefield Oct 16 '12 at 12:43