Javascript function challenge add(1,2) and add(1)(2) both should return 3

Question

A friend of mine challenged me to write a function that works with both of these scenarios

add(2,4)  // 6
add(2)(4) // 6

My instinct was the write an add() function that returns itself but I'm not sure I'm heading in the right direction. This failed.

function add(num1, num2){
    if (num1 && num2){
        return num1 + num2;
    } else {
        return this;
    }
}

alert(add(1)(2));

So I started reading up on functions that return other functions or return themselves.

• http://davidwalsh.name/javascript-functions
• JavaScript: self-calling function returns a closure. What is it for?
• JavaScript: self-calling function returns a closure. What is it for?

I am going to keep trying, but if someone out there has a slick solution, I'd love to see it!

Answer 1

I wrote a curried function whose valueOf() method and function context (this) are bound with the sum no matter how many arguments are passed each time.

/* add function */
let add = function add(...args) {
  const sum = args.reduce((acc, val) => acc + val, this);
  const chain = add.bind(sum);
  chain.valueOf = () => sum;
  return chain;
}.bind(0);

/* tests */
console.log('add(1, 2) = ' + add(1, 2));
console.log('add(1)(2) = ' + add(1)(2));
/* even cooler stuff */
console.log('add(1, 2)(3) = ' + add(1, 2)(3));
console.log('add(1, 2, 3)(4, 5)(6) = ' + add(1, 2, 3)(4, 5)(6));
/* retains expected state */
let add7 = add(7);
console.log('let add7 = add(7)');
console.log('add7(3) = ' + add7(3));
console.log('add7(8) = ' + add7(8));