Question presented to candidate: "Write a function add such that add(1)(2)(3) eventually evaluates to 6 -- and the number of calls is not fixed, so add(1)(2)(3)(4)(5) should evaluate to 15. How would you implement that, and how does the runtime actually know when to stop chaining and produce a number?"
What a strong answer should cover:
- Currying in general means turning a function that takes multiple arguments into a chain of functions that each take one argument (or a group), returning a new function until enough arguments have arrived.
- Infinite currying cannot rely on counting a fixed number of calls, because there is no fixed arity -- instead, each call returns another callable function that also carries an overridden valueOf/toString, so the chain can be extended indefinitely.
- The chain never "auto-terminates" by itself -- JavaScript only calls valueOf/toString when something coerces the returned function to a primitive, such as the + operator, a template literal, or JSON.stringify. Until that coercion happens, add(1)(2)(3) is still just a function sitting there.
- An alternative, more explicit style avoids relying on implicit coercion entirely: return a callable object with a distinct terminator method, such as .call() or .done(), that the caller invokes explicitly to end the chain -- more verbose, but avoids the somewhat surprising, coercion-only quirk that a plain console.log(add(1)(2)(3)) prints a function, not a number.
- Each intermediate function must close over the running total via a closure, so the accumulated sum persists across calls.
Clarifying questions expected:
- "Should the terminator be implicit, through coercion like plus or a template literal, or should I add an explicit terminator method the caller has to call?"
- "Does this need to support passing multiple arguments per call, like add(1, 2)(3), or strictly one argument per call?"
Code / implementation expected: Yes -- a real, executed implementation using the valueOf/toString coercion trick, plus a contrasting explicit-terminator version, both actually run and confirmed to produce correct sums for chains of varying length and grouping.