JS - Function Recursion

function add(a){
    var sum = function(b){
        return add(a + b);
    };

    sum.valueOf = sum.toString = function() {
        return a;
    };

    return sum;
}

console.log(add(1));
console.log(add(2)(3));
console.log(add(1)(1)(1)(1)(1));
console.log(add(1)(0)(-1)(-1));

var addTwo = add(2);
console.log(addTwo);
var addTwo = add(2);
console.log(addTwo(3));
var addTwo = add(2);
console.log(addTwo(3)(5));