assertNever Utility

export function assertNever(x: never): never {
  throw new Error("Unexpected object: " + x);
}

This tiny function is super helpful when working with discriminated unions in TypeScript. It ensures that every possible case is handled — and if a new case is added but not accounted for, you'll get a compiler error (or a runtime exception in development).

Example Usage

type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "square"; sideLength: number };

function getArea(shape: Shape): number {
  switch (shape.kind) {
    case "circle":
      return Math.PI * shape.radius ** 2;
    case "square":
      return shape.sideLength ** 2;
    default:
      return assertNever(shape);
  }
}

This pattern helps catch bugs early when you forget to handle a new kind. TypeScript will complain if shape is not of type never in the default case — meaning you've missed a case in the switch.

Tip: You’ll appreciate this the next time you refactor a union type!