A Javascript function that does recursive sum and has funny syntax (e.g. sum(3)(5)(-1)() // result 7)

sum.js

/**
 * A function that takes an abitrary number of parameters of the form sum(param1)(param2)(param3)...(paramN)()
 * and return the total sum = param1 + param2 + .. + paramN.
 * 
 * Note that the last parameter has to be empty. Thus, sum() is zero.
 **/
var sum = function(param) {
    var total = 0;

    var recursiveSum = function(p) {
        if( null == p ) {
            return total;
        }

        total += p;
        return recursiveSum;
    }

    return recursiveSum(param);
}

console.log(sum(-1)(3)(7)()) // output 9