How to create the function to give that result

Question

The function that will take over all the arguments needs to be created, and I don't understand what part of [1].a(3)

const add = () => {

}
add(1)(2)[1].a(3) // 1+2+3 = 6

Answer 1

const add = (first) => {
  return (second) => {
    return [
      undefined,
      {
        a: (third) => first + second + third
      }
    ];
  }
}

let result = add(1)(2)[1].a(3);
console.log(result);

:)