Question

JavaScript’s index is 32 bit, so it seems that the array index should be able to go up to 4294967295 for a total of 4294967296 elements. But in fact the highest index is 4294967294. Since an array has a length property, I don’t see a reason for having a null as the last element. Is there a reason the maximum index is 4294967294 but not 4294967295?

Answer 1

This is because when you create an array using the Array constructor you may supply it an optional length as follows:

new Array(length);

The length of an array is a 32-bit unsigned integer. Hence the length of the array may range from 0 to Math.pow(2, 32) - 1 which is 4294967295.

For an array of length n the indices range from 0 to n - 1. Hence the maximum index of a JavaScript array is (Math.pow(2, 32) - 1) - 1 or Math.pow(2, 32) - 2, which is 4294967294.

Thus a JavaScript array may hold a maximum of 4294967295 elements and not 4294967296 elements.

I know. It’s pretty illogical, but then again one element won’t make a lot of difference.