var foo = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20],
    find = [1, 10, 20],
    b = 21,
    i = find.length,
    indexOfUsingWhile = function (needle, haystack) {
        var j = haystack.length;
        while (j) {
            j -= 1;
            if (haystack[j] === needle) {
                return j;
            }
        }
        return -1;
    };

while (i--) {
    console.log(indexOfUsingWhile(find[i], foo));
}
i = find.length;
while (i--) {
    console.log(foo.indexOf(find[i]));
}
i = find.length;
while (i--) {
    console.log(indexOfUsingWhile(b, foo));
}
i = find.length;
while (i--) {
    console.log(foo.indexOf(b));
}