javascript - Array Out of Bounds: Comparison with undefined, or length check?

Question

this seems to be a common javascript idiom:

function foo (array, index) {
    if (typeof array[index] == 'undefined')
        alert ('out of bounds baby');
}

as opposed to the more prevalent (in other languages) and conceptually simpler:

function foo (array, index) {
    if (index >= array.length)
        alert ('boo');
}

I understand that the first case will also work for arrays which have 'gaps' in them, but is that a common enough case to warrant the idiom?

The code sample that prompted this question can be seen here. In this case, when using the 'argument' variable inside a function, isn't it sane to assume that it will be a contiguous array?

Answer 1

The only correct way is to check the index vs. the length.

An element may be assigned the value undefined. It is just silly to use it for a sentinel here. (There may be other, valid and possibly overlapping, reasons for checking for undefined, but not "for an out of bound check" -- the code in the other question will present arguably wrong results when the value of the given arg is really undefined.)

Happy coding.