Never

Never

The never type represents the type of values that never occur. For instance, never is the return type for a function expression or a arrow function expresssion that always throws an exception or one that never returns; Variables also acquire the type never when narrowed by any type guards that can never be true.

The never type is a subtype of, and assignable to, every type; however, no type is a subtype of, or assignable to, never (except never itself). Even any isn’t assignable to never.

Some examples of functions returning never:

// Function returning never must have unreachable end point
function error(message: string): never {
  throw new Error(message);
}

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

// Function returning never must have unreachable end point
function infiniteLoop(): never {
  while (true) {
  }
}