[RxJS] Implement the `map` Operator from Scratch in RxJS

While it's great to use the RxJS built-in operators, it's also important to realize you now have the knowledge to write them by yourself if needed. The mapoperator turns out to be a simple MapSubscriber which takes a function and applies it to the value passed to next.

map.js:

import { Subscriber } from "rxjs";

class MapSubscriber extends Subscriber {
  constructor(sub, fn) {
    super(sub);
    this.fn = fn;
  }

  _next(value) {
    this.destination.next(this.fn(value));
  }
}

export const map = fn => source =>
  source.lift({
    call(sub, source) {
      source.subscribe(new MapSubscriber(sub, fn));
    }
  });

multiply.js:

import { map } from "./map";

export const mul = number => map(v => v * number);

index.js:

import { from, Subscriber } from "rxjs";
import { multiply, mul } from "./multiply";

const observable$ = from([, , , , ]);

const subscriber = {
  next: value => {
    console.log(value);
  },
  complete: () => {
    console.log("done");
  },
  error: value => {
    console.log(value);
  }
};

observable$.pipe(mul()).subscribe(subscriber);
observable$.pipe(mul()).subscribe(subscriber);