custom rxjs

demo.ts

// Exemplo de uso:

of(10)
  .pipe(
    map((value) => value * 4),
    filter((value) => value > 2)
  )
  .subscribe((value) => console.log(value));

filter.ts

function filter<T>(predicate: (value: T) => boolean): Operator<T, T | null> {
  return (source: T) => (predicate(source) ? source : null);
}

map.ts

function map<T, R>(transform: (value: T) => R): Operator<T, R> {
  return (source: T) => transform(source);
}

observable.ts

type Subscriber<T> = (value: T) => void;
type Operator<T, R> = (source: T) => R;

class Observable<T> {
  constructor(private _producer: (subscriber: Subscriber<T>) => void) {}

  subscribe(next: Subscriber<T>) {
    this._producer(next);
  }

  pipe<R>(...operators: Operator<T, R>[]): Observable<R> {
    return operators.reduce(
      (observable, operator) =>
        new Observable((subscriber) =>
          observable.subscribe((value) => subscriber(operator(value)))
        ),
      this as Observable<any>
    ) as Observable<R>;
  }
}

of.ts

function of<T>(value: T): Observable<T> {
  return new Observable((subscriber) => subscriber(value));
}