TypeScript Practice 10/12

Exhaustiveness with never

Last lesson's area handled circles and rects. Six months from now someone adds triangle to the Shape union - and every switch over shapes in the codebase is suddenly incomplete. The dangerous version fails silently: the new shape falls through, returns undefined, and the bug surfaces three screens later as NaN.

The fix is the assertNever pattern: a default branch that passes the value to a function declared to accept never - the type with no values:

function assertNever(value: never): never {
  throw new Error("Unhandled kind: " + JSON.stringify(value));
}

It works on two levels. At compile time, if every case is handled, the value reaching default has type never, and the call type-checks; miss a case and the leftover type doesn't fit never - a red squiggle right at the switch. At runtime (which is what this playground grades - remember, types are stripped before execution), anything unhandled hits the default and throws immediately, with the offending value in the message. Loud and close to the cause, instead of silent and far away.

Your task: the Shape union has grown a triangle (base, height - area is base * height / 2). The switch doesn't handle it yet, so triangles currently fall into assertNever and throw. Add the triangle case. The default stays - it's the tripwire for the next shape someone adds.

You'll practice:

Reading and keeping the assertNever exhaustiveness tripwire
Extending a discriminated union switch without losing its safety net

type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "rect"; width: number; height: number }
  | { kind: "triangle"; base: number; height: number };

function assertNever(value: never): never {
  throw new Error("Unhandled kind: " + JSON.stringify(value));
}

function area(s: Shape): number {
  switch (s.kind) {
    case "circle":
      return Math.PI * s.radius * s.radius;
    case "rect":
      return s.width * s.height;
    // triangle is missing - right now it falls through to assertNever and throws
    default:
      return assertNever(s as never);
  }
}

const triArea = area({ kind: "triangle", base: 6, height: 4 });