Array equality checking algorithm not working in certain cases

Question

I have a piece of code which checks for equality of arrays. It works like a charm when using it like:

[1, 2, 3].equals([1, 2, 3]); // true
[1, 2, 3].equals([1, 2, 4]); // false

The above result is obvious and correct, of course. However, the following case fails:

[1, 2, undefined].equals([1, 2, undefined]);
// Error:  Cannot read property 'equals' of undefined

What could be the cause of it? I check whether it has the property before using it (if (this[i].equals)), so why does it say this? It is also true that undefined === undefined, so I do not see what's the problem.

The function:

Array.prototype.equals = function(arr) {
    if (this.length !== arr.length) {
        return false;
    }

    for (var i = 0; i < arr.length; i++) {
        if (this[i].equals) {
            if (!this[i].equals(arr[i])) {
                return false;
            } else {
                continue;
            }
        }

        if (this[i] !== arr[i]) {
            return false;
        }
    }

    return true;
}

Answer 1

This line:

    if (this[i].equals) {

should be:

    if (this[i] && this[i].equals) {

so that if this[i] is undefined, you won't be trying to access a property on undefined.

---

EDIT: Since you seem to be testing for an Array, it will be safer to do a more explicit test, since any Object could potentially have an .equals property.

You may want to do this instead:

if( Object.prototype.toString.call( this[i] ) === '[object Array]' ) {

It's a bit longer, but will accurately detect an Array, even if it was created using the new keyword, which would throw off the typeof operator.