Testing Result code

Test success, every expected error variant, short-circuiting, retries, and Panic behavior through public interfaces.

Assert the discriminated shape

const result = parsePort("3000");

expect(result.status).toBe("ok");
if (result.status === "ok") {
  expect(result.value).toBe(3000);
}

Narrow before accessing payloads so tests follow the same contract as production callers.

Test each error variant

const result = parsePort("nope");

expect(result.status).toBe("error");
if (result.status === "error") {
  expect(InvalidPort.is(result.error)).toBe(true);
  expect(result.error.input).toBe("nope");
}

For a workflow, cover every tagged variant the public signature promises—not only a generic failure assertion.

Test short-circuit behavior

const save = vi.fn(() => Result.ok(undefined));
const result = Result.gen(function* () {
  yield* Result.err(new InvalidInput({ message: "invalid" }));
  yield* save();
  return Result.ok(undefined);
});

expect(Result.isError(result)).toBe(true);
expect(save).not.toHaveBeenCalled();

Test retry policy deterministically

Inject or faithfully control the operation’s sequence. Assert attempt numbers and final Result. Keep delays tiny or use your test runner’s fake timers.

const attempts: number[] = [];
const result = await Result.tryPromise(
  async ({ attempt }) => {
    attempts.push(attempt);
    if (attempt < 3) throw new Error("temporary");
    return "ready";
  },
  { retry: { times: 2, delayMs: 0, backoff: "constant" } },
);

expect(attempts).toEqual([1, 2, 3]);
expect(Result.isOk(result)).toBe(true);

Assert defects separately

expect(() =>
  Result.ok(1).map(() => {
    throw new Error("bug");
  }),
).toThrow(Panic);

Do not make expected-error tests catch Panic; that blurs the contract you are trying to prove.

Add compile-time tests

Inference is part of the API. Use your repository’s type-test convention to verify important unions:

const result = Result.all([Result.ok(1), Result.err("failed" as const)] as const);
type Error = InferErr<typeof result>; // "failed"