Observable2.ts

/**
 * A simple object with a `next` and `complete` callback on it.
 */
interface Observer<T> {
  next: (value: T) => void;
  complete: () => void;
}
 
/**
 * A function that takes a simple object with callbacks
 * and does something them.
 */
const source = (subscriber: Observer<number>) => {
  subscriber.next(1);
  subscriber.next(2);
  subscriber.next(3);
  subscriber.complete();
};
 
// Usage
console.log('start');
source({
  next: console.log,
  complete: () => console.log('done'),
});
console.log('stop');