assertNever

export function assertNever(x: never, failOnRuntime?: boolean): never {
  console.error("assertNever", x);
  if (failOnRuntime) {
    throw new Error("Unreachable point reached.");
  }
}

Example

The example was taken from aikoven/assert-never.

type A = { type: "a" };
type B = { type: "b" };
type Union = A | B;

function doSomething(arg: Union) {
  if (arg.type === "a") {
    return something;
  }

  if (arg.type === "b") {
    return somethingElse;
  }

  // TypeScript typechecker will error if there are other types in the union.
  // Will not throw on runtime.
  assertNever(arg);

  // TypeScript typechecker will error if there are other types in the union.
  // Will throw on runtime.
  assertNever(arg, true);
}