Frontend Curry: addTwoNumbers → addThreeNumbers(a)(b)(c)

Salesforce's recurring fullstack phone-screen warm-up. Given `addTwoNumbers(a, b) = a + b`, implement a curried `addThreeNumbers(a)(b)(c)` that returns `a + b + c`. Frequently extended into a generic `curry` or `addN` variadic version. Requirements Implement addThreeNumbers such that addThreeNumbers(1)(2)(3) === 6. Same (arg)(arg)(arg) chain shape — each call returns either another function (more args expected) or the final number. JavaScript / TypeScript only; no React allowed in CoderPad. Common follow-ups asked in the same round: Generalise to addN(a)(b)(c)... for arbitrary arity (i.e. the curried function is variadic until called with zero args, where it returns the sum). Implement a general curry(fn) that transforms any fixed-arity fn into its curried form. Examples addThreeNumbers(1)(2)(3); // 6 addThreeNumbers(10)(20)(30); // 60 // follow-up: generic curry const sum3 = curry((a, b, c) => a + b + c); sum3(1)(2)(3); // 6 sum3(1, 2)(3); // 6 (curry should accept partial application) sum3(1)(2, 3); // 6 Notes The 3-arg version is one-liner: const addThreeNumbers = a => b => c => a + b + c;. Generic curry: function curry(fn) { return function curried(...args) { if (args.length >= fn.length) return fn.apply(this, args); return (...next) => curried.apply(this, [...args, ...next]); }; } Uses fn.length (arity declared by the parameter list) as the terminator. The variadic / infinite-add version uses an explicit zero-argument call as its terminator: function addN(initial) { let total = initial; function next(...args) { if (args.length === 0) return total; total += args.reduce((sum, value) => sum + value, 0); return next; } return next; } addN(1)(2)(3)(); // 6 A valueOf / toString coercion variant is a separate contract; only use it when the interviewer explicitly asks for arithmetic coercion instead of () termination. This is often paired with a traffic-light frontend exercise in the same screen — practise both back-to-back. Preparation Memorise the three forms — fixed-3-arg, generic curry, infinite-add — and be able to switch between them based on the interviewer's follow-up. Practise typing the generic curry from scratch in under 3 minutes. Drill a generic curry exercise that accepts mixed partial application such as (1, 2)(3) and (1)(2, 3). Pre-test on coderpad-like environments because import / require behaviour and console output differ from a local Node REPL.