TypeScript discriminated union exhaustive check

When handling a discriminated union, there's no compile-time guarantee that all cases are covered. Adding a new variant can silently miss handlers.

Use the never type for exhaustive checking:

type Shape =
| { kind: 'circle'; radius: number }
| { kind: 'square'; side: number }
| { kind: 'triangle'; base: number; height: number };

// Exhaustive helper
function assertNever(x: never): never {
throw new Error(Unexpected value: ${JSON.stringify(x)});
}

function area(shape: Shape): number {
switch (shape.kind) {
case 'circle': return Math.PI * shape.radius ** 2;
case 'square': return shape.side ** 2;
case 'triangle': return (shape.base * shape.height) / 2;
default: return assertNever(shape); // Compile error if case missing!
}
}

// If you add { kind: 'pentagon'; ... } to Shape,
// the switch will get a compile error at assertNever
// because 'pentagon' doesn't match 'never'.

The never type represents values that should never occur. If TypeScript can reach assertNever, a case was missed, causing a compile error.