type script optionals

optional.ts

export declare type Optional<T> = T | undefined;
  
export function getOptional<T>(optional: Optional<T>, defaultVal?: T): T {
  if (optional) {
    return <T>optional;
  }
  if (defaultVal != undefined) {
    return defaultVal;
  }
  throw new Error("the value of optional is undefined");
}

const foo: string | undefined = "foo";

console.log(getOptional(foo)); // => "foo"