The `never` Type: A Value That Should Never Exist

WHY IT EXISTS: TypeScript needed a way to represent impossible states and unreachable code paths at the type level. This allows the compiler to perform more robust static analysis, like ensuring all cases in a discriminated union are handled, which makes code safer and more maintainable.

THE MENTAL MODEL: Think of never as the empty set of types. It represents a value that should never, ever occur. If a variable is typed as never, it means something has gone logically wrong in your type logic, because no value can be assigned to it.

HOW IT WORKS: never is a "bottom type." This means it is assignable to every other type, but no other type (except never itself) is assignable to it. A function can only have a never return type if it guarantees it will not have a reachable end point. This is typically achieved in two ways: by always throwing an exception or by entering an infinite loop.

WHEN TO USE IT: The primary use case is for exhaustive checking in switch statements or conditional logic. By assigning the final unhandled case to a never variable, you force the compiler to error if a new type is added to a union without being handled. It is also the correct return type for functions designed to always throw an error, clearly signaling their non-returning behavior to callers.

WHEN NOT TO USE IT: Do not use never when you mean void. A function that completes its execution but doesn't return a value (like console.log) has a return type of void. never is exclusively for functions or code paths that do not complete and never return control to the caller.

ONE CANONICAL EXAMPLE: Exhaustive checks with a discriminated union are the classic example. Consider a function handling different shapes: type Shape = "circle" | "square"; function getArea(s: Shape) { switch(s) { case "circle": return 1; case "square": return 2; default: const _exhaustiveCheck: never = s; return _exhaustiveCheck; } }. If you later add "triangle" to the Shape type, TypeScript will show an error on the _exhaustiveCheck line because the type "triangle" cannot be assigned to never, forcing you to handle the new case.