Assertion Functions (asserts keyword) for Invariant Checking

Validation functions that throw on failure don't narrow the type after the call — TypeScript still treats the value as potentially invalid even though control flow proves it passed.

Use the 'asserts' return type to tell TypeScript that if the function returns normally, the condition holds.

function assertDefined<T>(val: T): asserts val is NonNullable<T> {
  if (val === null || val === undefined) {
    throw new Error(`Expected defined, got ${val}`);
  }
}

function assertString(val: unknown): asserts val is string {
  if (typeof val !== 'string') {
    throw new TypeError(`Expected string, got ${typeof val}`);
  }
}

const maybeUser: User | undefined = getUser();
assertDefined(maybeUser);
// Now TypeScript knows maybeUser is User (not undefined)
maybeUser.name; // No error

// Also works with Node's built-in assert:
import assert from 'node:assert';
assert(typeof value === 'string');
value.toUpperCase(); // narrowed to string

When a function has 'asserts condition' as its return type, TypeScript models the post-call state as the condition being true — similar to a type guard but for the unconditional path.