Result Types Instead of Throwing

Example

type Result<T, E> = { ok: true; value: T } | { ok: false; error: E };
const Ok = <T>(value: T): Result<T, never> => ({ ok: true, value });
const Err = <E>(error: E): Result<never, E> => ({ ok: false, error });

type CouponError =
  | { type: "NOT_FOUND" }
  | { type: "EXPIRED"; on: string }
  | { type: "MIN_ORDER"; min: number };

const coupons: Record<string, { percent: number; min: number; expires: string }> = {
  FEST20: { percent: 20, min: 999, expires: "2026-12-31" },
  OLD10: { percent: 10, min: 0, expires: "2025-01-01" },
};

function applyCoupon(code: string, cart: number, today: string): Result<number, CouponError> {
  const c = coupons[code];
  if (!c) return Err({ type: "NOT_FOUND" });
  if (today > c.expires) return Err({ type: "EXPIRED", on: c.expires });
  if (cart < c.min) return Err({ type: "MIN_ORDER", min: c.min });
  return Ok(cart - (cart * c.percent) / 100);
}

for (const [code, cart] of [["FEST20", 1500], ["FEST20", 500], ["OLD10", 800], ["XYZ", 800]] as const) {
  const r = applyCoupon(code, cart, "2026-01-15");
  if (r.ok) console.log(code, "-> pay", r.value);
  else if (r.error.type === "MIN_ORDER") console.log(code, "-> add Rs.", r.error.min - cart, "more");
  else if (r.error.type === "EXPIRED") console.log(code, "-> expired on", r.error.on);
  else console.log(code, "-> invalid coupon");
}

Output

FEST20 -> pay 1200
FEST20 -> add Rs. 499 more
OLD10 -> expired on 2025-01-01
XYZ -> invalid coupon

Example

Reading value without checking ok is an error (TS2339), because the failure branch has no value.

type Result<T, E> = { ok: true; value: T } | { ok: false; error: E };
declare function parseAmount(s: string): Result<number, string>;

const total = parseAmount("100").value + 10;

Compiler output (tsc)

Line 4: error TS2339: Property 'value' does not exist on type 'Result<number, string>'.
  Property 'value' does not exist on type '{ ok: false; error: string; }'.