Assertion Functions in TypeScript

function assert(condition: unknown, message: string): asserts condition {
  if (!condition) throw new Error(message);
}

function greet(name: unknown) {
  assert(typeof name === "string", "Name must be a string");
  name.toUpperCase(); // name is now string
}

function assertIsString(value: unknown): asserts value is string {
  if (typeof value !== "string") throw new Error("Not a string");
}

function processValue(value: unknown) {
  assertIsString(value);
  value.toUpperCase(); // value is string
}

class UserValidator {
  static assertValidUser(data: unknown): asserts data is User {
    if (!data || typeof data !== "object") throw new Error("Invalid user");
    if (!("name" in data) || typeof (data as any).name !== "string") throw new Error("Name required");
    if (!("email" in data) || typeof (data as any).email !== "string") throw new Error("Email required");
  }
}

function handleRequest(body: unknown) {
  UserValidator.assertValidUser(body);
  body.name.toUpperCase(); // body is now User
}