index.ts

export function assertNever(value: never, errorMessage?: string): never
export function assertNever(value: never, getErrorMessage?: (value: unknown) => string): never
export function assertNever(value: never, noThrow?: boolean): never

/**
 * Helper function for exhaustive checks of discriminated unions.
 * https://basarat.gitbooks.io/typescript/docs/types/discriminated-unions.html
 *
 * @example
 *
 *    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;
 *      }
 *
 *      // TS will error if there are other types in the union
 *      // Will throw an Error when called at runtime.
 *      // Use `assertNever(arg, true)` instead to fail silently.
 *      return assertNever(arg);
 *    }
 */
export function assertNever(value: never, errorMessageOrNoThrow?: string | ((value: unknown) => string) | boolean): never {
  if (typeof errorMessageOrNoThrow === 'string') {
    throw new Error(errorMessageOrNoThrow)
  }

  if (typeof errorMessageOrNoThrow === 'function') {
    throw new Error(errorMessageOrNoThrow(value))
  }

  if (errorMessageOrNoThrow) {
    return value
  }

  throw new Error(
    `Unhandled discriminated union member: ${JSON.stringify(value)}`,
  );
}

export default assertNever;