Currying function with unknown arguments in JavaScript

Question

In a recent interview, I was asked to write a function that adds numbers and accepts parameters like this:

add(1)(2)(3) // result is 6
add(1,2)(3,4)(5) // re         


        

Answer 1

I'm a bit late to the party, but something like this would work (a bit hacky though in my opinion):

const add = (a, ...restA) => {
  const fn = (b, ...restB) => {
    return add([a, ...restA].reduce((x, y) => x + y) + [b, ...restB].reduce((x, y) => x + y))
  };
  fn.valueOf = () => {
    return [a, ...restA].reduce((x, y) => x + y)
  };
  return fn;
}

This function returns a function with a value of the sum. The tests below are outputing the coerced values instead of the actual functions.

console.log(+add(1,2)(3,4)(5)); // 15
console.log(+add(1)) // 1
console.log(+add(1)(2)) // 3
console.log(+add(1)(2)(3)) // 6
console.log(+add(1)(2)(3)(4)) // 10

Since it's a currying function, it will always return another function so you can do something like this:

const addTwo = add(2);
console.log(+addTwo(5)); // 7