Why is a Javascript Array index at most 4294967294 but not 4294967295?

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 4

Answer 1

The ECMA-262 specification (section 15.4) says:

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 2³²-1.

The spec also says that the length property of an array is always less than 2³². That would seem to exclude 4294967295 as an array index.