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Coding problem · Medium · Language Idioms

Result Type Instead of Exceptions

Medium Language practicals Seen at Stripe, Vercel, Linear

Define type Result<T, E> = { ok: true; value: T } | { ok: false; error: E } and implement:

  1. parseAge(input: string): Result<number, string>
    • trim the input; it must be digits only, otherwise { ok: false, error: "not a number" };
    • the value must be between 0 and 150 inclusive, otherwise { ok: false, error: "out of range" }.
  2. tryCatch<T>(fn: () => T): Result<T, Error> — run fn; return ok with its value, or ok: false with the thrown value (wrap non-Error throws in an Error).

Hints

Hint 1

The ok literal type is the discriminant: after if (r.ok), TypeScript knows r.value exists.

Hint 2

A regex like /^\d+$/ rejects -5, 4.2 and ''.

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Solution and explanation

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Time O(length of input) · Space O(1)

A discriminated union makes failure part of the return type, so callers must handle it — the compiler narrows r.value vs r.error after checking r.ok. This is the style of Rust's Result, and of libraries like neverthrow.

catch (e) gives you unknown under strict (useUnknownInCatchVariables), so normalise it into an Error before returning it.

Reference solution

type Result<T, E> = { ok: true; value: T } | { ok: false; error: E };

function parseAge(input: string): Result<number, string> {
  const text = input.trim();
  if (!/^\d+$/.test(text)) return { ok: false, error: "not a number" };
  const age = Number(text);
  if (age > 150) return { ok: false, error: "out of range" };
  return { ok: true, value: age };
}

function tryCatch<T>(fn: () => T): Result<T, Error> {
  try {
    return { ok: true, value: fn() };
  } catch (e) {
    return { ok: false, error: e instanceof Error ? e : new Error(String(e)) };
  }
}

Common mistakes

  • Number("") is 0 and parseInt("12abc") is 12 — validate the text first.
  • Assuming the caught value is an Error.

Follow-up questions

  • Add map and andThen helpers.
  • Make tryCatchAsync for promises.