>> Arrays cannot have holes. Writing an element after the end is not allowed:

    a = []
    a[0] = 1; // OK to extend the array length
    a[10] = 2; // TypeError

Guess I'm a bit fuzzy on this, I wouldn't use numeric keys to populate a "sparse array", but why would it be a problem to just treat it as an iterable with missing values undefined? Something to do with how memory is being reserved in C...? If someone jumps from defining arr[0] to arr[3] why not just reserve 1 and 2 and inform that there's a memory penalty (ie that you don't get the benefit of sparseness)?