What is the never type used for?

never is the bottom type. No value is assignable to never except another never, and never is assignable to every type. The compiler uses it to represent code paths that cannot be reached or functions that never return normally.

function fail(message: string): never {
  throw new Error(message);
}

type Result = string | never;

function assertNever(value: never): never {
  throw new Error("Unexpected value: " + String(value));
}

Where never appears

• Functions that always throw or loop forever
• Exhaustive switch defaults after every union member is handled
• Conditional types that filter out non-matching members
• Intersections with incompatible properties
• Unreachable branches after narrowing

In a union, never disappears because it contributes no values. That property is what makes exhaustive checks work: after every real member is handled, the remaining value is never, so passing it to a function that accepts never compiles only if the switch is complete.