EventBus.js

/*
You have 2 different applications running in the webpage, one with the cart count and another is a list of products. When you add click on "Add to cart" in the project, the cart count needs to get updated. 

Implement a eventBus / pub/Sub solution to allow communication between the two applications.

Write tests to cover common use cases.
*/

const EventBus = () => {
  const listeners = {}

  const subscribe = (eventName, callback) => {
    if (typeof callback !== 'function') throw Error()

    if (!(eventName in listeners)) {
      listeners[eventName] = []
    }

    listeners[eventName].push(callback)

    return () => {
      unsubscribe(eventName, callback)
    }
  }

  const once = (eventName, callback) => {
    const callbackWrapper = (...args) => {
      unsubscribe(eventName, callbackWrapper)
      callback(...args)
    }

    return subscribe(eventName, callbackWrapper)
  }

  const publish = (eventName, ...args) => {
    if (!(eventName in listeners)) return

    listeners[eventName].forEach(cb => cb(...args))
  }

  const unsubscribe = (eventName, callback) => {
    listeners[eventName] = listeners[eventName].filter(cb => callback !== cb)
  }

  return {
    subscribe,
    once,
    publish,
  }
}


const eventbus = EventBus()
const unsubscribe = eventbus.once('add-to-cart', (product, timestamp) => console.log(product, timestamp))
eventbus.publish('add-to-cart', {id:1}, 123123)
unsubscribe()
eventbus.publish('add-to-cart', {id:1}, 123123)



const pubsub = EventBus();

const fn1 = (data) => console.log("subscribe 2", data);
pubsub.subscribe("eventA", (data) => console.log("subscribe 1", data));
pubsub.subscribe("eventA", fn1);

pubsub.publish("eventA", {details: {}});

pubsub.unsubscribe("eventA", fn1)

pubsub.publish("eventA", {details: {}});