ts-expect

import { expectType, TypeEqual, expectNever } from "ts-expect"; // --- Basic expectType assertions --- // These checks are performed at compile-time by TypeScript. expectType<string>("hello world"); expectType<number>(42); // The following line would cause a compile-time error: // expectType<boolean>("true"); // Type 'string' is not assignable to type 'boolean'. // --- Using TypeEqual for stricter type comparisons --- // TypeEqual is a utility type to ensure two types are exactly the same. interface User { id: string; name: string; } type AdminUser = { id: string; name: string; roles: string[] }; // Expects that User and AdminUser are NOT equal, so asserting 'false' should pass. expectType<TypeEqual<User, AdminUser>>(false); // The following would cause a compile-time error if types were not equal: // expectType<TypeEqual<User, { id: string; name: string }>>(false); // Type 'false' is not assignable to type 'true'. // --- Exhaustive checks with expectNever --- // expectNever is useful for ensuring all cases in a discriminated union or switch statement are handled. type TrafficLight = "red" | "yellow" | "blue"; // Typo: 'blue' should likely be 'green' function getAction(light: TrafficLight): string { switch (light) { case "red": return "Stop"; case "yellow": return "Prepare to stop"; // If 'green' was intended and 'blue' added, this 'default' will catch unhandled types. default: // This line will trigger a TypeScript error if 'light' is not assignable to 'never'. // It helps catch unhandled cases in unions at compile time. return expectNever(light); // Expects 'light' to be 'never' here. } } getAction("red"); getAction("yellow"); // The 'blue' case will cause a compile-time error due to expectNever if it's not handled. // This demonstrates a powerful pattern for ensuring type safety in complex logic.