Assertion Functions (asserts val is T)

TypeScript supports a special kind of function that asserts a condition. If the function returns without throwing, TypeScript narrows the type:

function assertDefined<T>(
  val: T | null | undefined,
): asserts val is NonNullable<T> {
  if (val == null) throw new Error('Value is null or undefined');
}

let config: Config | null = loadConfig();
assertDefined(config);
config.apiKey; // TypeScript knows config is Config here ✅

The key difference from a type guard:

• Type guard (x is T): returns a boolean, narrowing happens in an if block
• Assertion function (asserts x is T): void return, narrowing happens after the call

Implement

function assertDefined<T>(
  val: T | null | undefined,
  message?: string,
): asserts val is NonNullable<T>

• Throws Error with message if val is null or undefined
• If no message provided, use a descriptive default
• Returns void on success (TypeScript infers the narrowing)

Note: NonNullable<T> is a built-in utility type that removes null | undefined from T.