13. Never

The never type represents values that never occur. It is used for functions that either throw an error or run indefinitely, meaning they never complete or return a value.

Example 1: Function that Throws an Error

function error(message: string): never {
  throw new Error(message);  // The function doesn't return, it throws an error
}

• A function that throws an error has the return type never, because it never completes successfully or returns a value.

Example 2: Inferred Return Type of never

function fail() {
  return error("Something failed");  // Inferred return type is `never`
}

• The return type of fail is inferred as never because it calls error, which never completes.

Example 3: Infinite Loop

function infiniteLoop(): never {
  while (true) {}  // Infinite loop, function never ends
}

• A function with an infinite loop also has a never return type because it never finishes and never returns a value.

Key Points:

• never is used for functions that never return, either because they throw an error or run indefinitely.
• A function with a never return type has an unreachable endpoint and will not complete its execution normally.