Currying Interview Questions in JavaScript

Currying Interview Questions

Q1: Implement sum(1)(2)(3)()

function sum(a) {
  return function (b) {
    if (b !== undefined) return sum(a + b);
    return a;
  };
}
sum(1)(2)(3)(); // 6

Q2: Implement sum(1)(2)(3)(4)..(n) (Amazon)

function sum(a) {
  return function (b) {
    if (b !== undefined) return sum(a + b);
    return a;
  };
}
sum(1)(2)(3)(4)(); // 10

Or with toString:

function sum(n) {
  const fn = (m) => sum(n + m);
  fn.toString = () => n;
  return fn;
}
sum(1)(2)(3) + 0; // 6

Q3: Implement a generic curry function

function curry(fn) {
  return function curried(...args) {
    if (args.length >= fn.length) return fn(...args);
    return (...next) => curried(...args, ...next);
  };
}

Q4: What is the difference between currying and partial application?

Currying: f(a)(b)(c) (one at a time). Partial: f(a, b) with c preset (some at once).

The Takeaway

Interview questions: sum(1)(2)(3)() (recursive with termination), sum(1)(2)(3)(4)..(n) (Amazon, same pattern), generic curry (recursive with fn.length check), and the difference between currying and partial application.