Sum of 'Z' numbers in an array

elastic.js

/*
 * Complete the function below.
 */
function isSumPossibleZ(numbers, expectedSum, numbersToSum) {
    function f(index, expectedSum, numbersToSum) {
        if (numbersToSum === 0) return expectedSum === 0;
        for (var length = numbers.length; index < length; index++) {
            if (f(index + 1, expectedSum - numbers[index], numbersToSum - 1)) {
                return 1;
            }
        }
        return 0;
    }
    numbers.sort();
    return f(0, expectedSum, numbersToSum);
}