[RxJS] Create a Reusable Operator from Scratch in RxJS

With knowledge of extending Subscriber and using source.lift to connect a source to a subscriber, you can now create your own operators by writing functions that return a source.lift call. This lesson creates a simple "multiply" operator in RxJS.

index.js:

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

const observable$ = from([1, 2, 3, 4, 5]);

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

observable$.pipe(multiply(3)).subscribe(subscriber);

multiply.js:

import { Subscriber } from "rxjs";

class MultiplySubscriber extends Subscriber {
  constructor(subscriber, number) {
    super(subscriber);
    this.number = number;
  }

  _next(value) {
    this.destination.next(value * this.number);
  }
}

export const multiply = number => source => {
  return source.lift({
    call(sub, source) {
      source.subscribe(new MultiplySubscriber(sub, number));
    }
  });
};

The most common scenario for creating custom operators is to reuse the built-in operators shipped with RxJS. You'll find yourself re-using map, filter, and others will solve most of the problems you come across.

import { map } from "rxjs/operators";
export const mul = number => map(v => v * number);