Exhaustive switch with assertNever
Make the compiler fail when a union gains a new member that a switch does not handle.
- TypeScript
- GreenSnippets
- MIT
- Executed with Node.js 22 (type stripping)
type Shape =
| { kind: "circle"; radius: number }
| { kind: "square"; side: number };
function assertNever(value: never): never {
throw new Error(`Unhandled variant: ${JSON.stringify(value)}`);
}
function area(shape: Shape): number {
switch (shape.kind) {
case "circle":
return Math.PI * shape.radius ** 2;
case "square":
return shape.side ** 2;
default:
return assertNever(shape);
}
}How it works
Calling assertNever in the default branch means the argument must have type never. If someone adds a new variant to the union and forgets a case, the build fails at that exact line. At runtime it throws, which also catches bad data that bypassed the types.
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