unknown, never, and Honest Boundaries
intermediate12 min readLesson 89 of 143
any vs unknown, narrowing unknown safely, never for impossible states, and where each belongs in a codebase.
any: the off switch
any opts out of checking — assignable to and from everything. It silences the
error today and reintroduces the runtime crash tomorrow. Every any in a
codebase is a hole in the contract.
unknown: the honest version
unknown accepts anything too — but you must narrow before use:
function parse(input: unknown) {
// input.foo ✗ — unknown is not usable yet
if (typeof input === "string") return input.trim(); // ✓ narrowed
if (isUser(input)) return renderUser(input); // ✓ guard
throw new Error("Unexpected payload");
}
Rule of thumb: unknown for anything that crosses a boundary you do not
control — network responses, JSON.parse, localStorage, third-party callbacks.
never: the impossible type
never is the type of values that cannot exist: a function that always throws,
or the exhaustiveness check from the unions lesson. When the compiler computes
"there are no remaining cases", the result is never — assigning it forces the
check.
Where each belongs
- Parameters you truly accept anything:
unknown+ narrow. - Values you construct that are impossible: design them away, or
never. any: only in genuinely untyped legacy seams, with a comment and a plan.