@lucid-softworks/invariant

A tiny assertion function with TypeScript narrowing and lazy error messages.

import { invariant } from "@lucid-softworks/invariant";

const user = users.get(id);
invariant(user, () => `User ${id} was not found`);

// `user` is narrowed here.

Failed assertions throw InvariantError. A message callback is evaluated only when the assertion fails.