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Result<T, E>: errors without exceptions

The Result<T, E> type is either a success { ok: true, value } or a failure { ok: false, error }. Write precisely typed helpers for it.

ok(value) and err(error) create a result, mapResult(result, fn) transforms a success value and passes a failure through unchanged (without calling fn), and unwrapOr(result, fallback) returns the value or the fallback. The error type should carry over to the result of mapResult.

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

function ok(value: any): any {
  // ...
}

function err(error: any): any {
  // ...
}

function mapResult(result: any, fn: (value: any) => any): any {
  // ...
}

function unwrapOr(result: any, fallback: any): any {
  // ...
}

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Tests (8)

Type tests are checked by the TypeScript compiler in strict mode. Expect<Equal<A, B>> passes when the types match exactly, and // @ts-expect-error passes when the next line does not compile. The first run downloads the compiler, so it takes a few seconds.

types

const r = ok(42);
type _ = Expect<Equal<typeof r, Result<number, never>>>;

types

const e = err('not found');
type _ = Expect<Equal<typeof e, Result<never, string>>>;

types

const r = {} as Result<number, string>;
const m = mapResult(r, (n) => String(n));
type _ = Expect<Equal<typeof m, Result<string, string>>>;

types

const r = {} as Result<number, string>;
const v = unwrapOr(r, 0);
type _ = Expect<Equal<typeof v, number>>;

expect(ok(1)).toEqual({ ok: true, value: 1 });
expect(err('x')).toEqual({ ok: false, error: 'x' });

expect(mapResult(ok(2), (n) => n * 10)).toEqual({ ok: true, value: 20 });

const f = fn((n) => n * 10);
const failure = err('bad');
expect(mapResult(failure, f)).toEqual({ ok: false, error: 'bad' });
expect(f).toHaveBeenCalledTimes(0);

expect(unwrapOr(ok(5), 0)).toBe(5);
expect(unwrapOr(err('bad'), 0)).toBe(0);