An `add(...)` function that keeps going forever

add.js

/**
 * Second Implementation
 * 
 * 1. The function `bind(a, b)` will return the calling function with the first
 * argument as context for, and the rest of the arguments as arguments to the
 * returned function.
 * 
 * 2. In addition to the bind call, the `valueOf` property on every function
 * type is the function that is called whenever the JS runtime needs to typecast
 * that function to a primitive type. The type of variable actually being
 * returned by the add function is a function, however when this function is
 * typecasted (by adding it to another number or string), the `valueOf` function
 * is called, and the result of that function is then used as the value of the
 * function.
 * 
 * In this implementation, typecasting is done using the unary plus (+) 
 * operator. It precedes its operand and evaluates to its operand but attempts 
 * to convert it into a number, if it isn't already.
 * 
 * Unary plus is the fastest and preferred way of converting something into a 
 * number, because it does not perform any other operations on the number.
 * — MDN (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Arithmetic_Operators)
 */

function add() {
    var sum = 0;

    for (var i in arguments) {
        sum += arguments[i];
    }

    var addAndRepeat = add.bind(null, sum);

    addAndRepeat.valueOf = function valueOf() {
        return sum;
    };

    return addAndRepeat;
}

console.log(+add()); //........................................ 0
console.log(+add(1)); //....................................... 1
console.log(+add(1, 2)); //.................................... 3
console.log(+add(1)(2)); //.................................... 3
console.log(+add(1, 2)(3)); //................................. 6
console.log(+add()()(1, 2)()(3)); //........................... 6
console.log(+add(1, 2)(3)(4, 5)(6)); //........................ 21
console.log(+add(1)(2)(3)(4)(5)(6)(7)(8)); //.................. 36

addUntilEmptyInvocation.js

/**
 * First Implementation
 * 
 * A parameter-less invocation of `add()` marks the end of operation
 */

function addUntilEmptyInvocation(x) {
    var sum = 0;

    sum += x;

    return function addAndRepeat(y) {
        if (typeof y !== 'undefined') {
            sum += y;
            return addAndRepeat;
        } else {
            return sum;
        }
    };
}

console.log(addUntilEmptyInvocation(1)(2)(3)(4)(5)(6)()); //... 21