Exhaustiveness checking

Exhaustiveness checking

We would like the compiler to tell us when we don’t cover all variants of the discriminated union. For example, if we add Triangle to Shape, we need to update area as well:

type Shape = Square | Rectangle | Circle | Triangle;
function area(s: Shape) {
  switch (s.kind) {
    case "square": return s.size * s.size;
    case "rectangle": return s.height * s.width;
    case "circle": return Math.PI * s.radius ** 2;
  }
  // should error here - we didn't handle case "triangle"
}

There are two ways to do this. The first is to turn on --strictNullChecks and specify a return type:

function area(s: Shape): number { // error: returns number | undefined
  switch (s.kind) {
    case "square": return s.size * s.size;
    case "rectangle": return s.height * s.width;
    case "circle": return Math.PI * s.radius ** 2;
  }
}

Because the switch is no longer exhaustive, TypeScript is aware that the function could sometimes return undefined. If you have an explicit return type number, then you will get an error that the return type is actually number | undefined. However, this method is quite subtle and, besides, --strictNullChecks does not always work with old code.

The second method uses the never type that the compiler uses to check for exhaustiveness:

function assertNever(x: never): never {
  throw new Error("Unexpected object: " + x);
}
function area(s: Shape) {
  switch (s.kind) {
    case "square": return s.size * s.size;
    case "rectangle": return s.height * s.width;
    case "circle": return Math.PI * s.radius ** 2;
    default: return assertNever(s); // error here if there are missing cases
  }
}

Here, assertNever checks that s is of type never — the type that’s left after all other cases have been removed. If you forget a case, then s will have a real type and you will get a type error. This method requires you to define an extra function, but it’s much more obvious when you forget it.