Observer pattern in javascript

main.js

const { randomUUID } = require("crypto")

/**
 * Create an observable value
 */
class Observable {
  observableValue
  subscribers = {}

  /**
   * Get current value
   */
  get value() {
    return this.observableValue
  }

  /**
   * Add a listener to changes in observableValue
   */
  addListener(listener) {
    const listenerId = randomUUID()
    this.subscribers[listenerId] = listener
    return listenerId
  }

  /**
   * Remove a subscriber by passing its refference
   */
  removeListener(listener) {
    switch (typeof listener) {
      case "function":
        for (let listenerId in this.subscribers) {
          if (this.subscribers[listenerId] === listener) {
            delete this.subscribers[listenerId]
          }
        }
        break
      case "string":
        delete this.subscribers[listener]
        break
      default:
        break
    }
  }

  notifySubscribers(id = undefined) {
    /*
     * If specifying a subscriber id
     */
    if (id) {
      switch (typeof id) {
        case "function":
          for (let subscriberId in this.subscribers) {
            if (this.subscribers[subscriberId] === id) {
              this.subscribers[subscriberId](this.value)
            }
          }
          break
        case "string":
          if (typeof this.subscribers[id] === "function") {
            this.subscribers[id](this.value)
          }
          break
        default:
          break
      }
    } else {
    /**
     * Otherwise notify all subscribers
     */
      for (let subscriberId in this.subscribers) {
        this.subscribers[subscriberId](this.value)
      }
    }
  }

  /**
   * Change observable value
   */
  set(newValue, subscribersToNotify) {
    let $this = this
    this.observableValue = newValue(this.value)
    /**
     * If provided with a list of subscribers
     */
    if (Array.isArray(subscribersToNotify)) {
      subscribersToNotify.forEach(function (subscriber) {
        for (let subscriberId in $this.subscribers) {
          if ($this.subscribers[subscriberId] === subscriber) {
            $this.notifySubscribers(subscriberId)
          }
        }
      })
    } else if ("function" === typeof subscribersToNotify) {
      /**
       * Notify by one subscriber reference
       */
      if ("function" === typeof subscribersToNotify) {
        for (let subscriberId in $this.subscribers) {
          if ($this.subscribers[subscriberId] === subscriberToNotify) {
            $this.notifySubscribers(subscriberId)
          }
        }
      }
    } else {
      /**
       * Otherwise, notify all subscribers
       */
      this.notifySubscribers()
    }
  }

  constructor(value) {
    this.observableValue = value
  }
}

Example:

function main() {
  const Counter = new Observable(0)

  // Will get added when Counter value is 4
  const allListenersRemoved = () => {
    console.log("No active listeners.")
  }

  const countListener = (count) => {
    // Remove listener
    if (count === 4) {
      console.log(`Count has changed to 4. Removing listener.`)
      Counter.removeListener(countListener)
      Counter.addListener(allListenersRemoved)
    } else console.log(`Count has changed to ${count}.`)
  }

  Counter.addListener(countListener)

  // Update value every 2 seconds
  setInterval(() => {
    Counter.set((c) => c + 1)
  }, 2000)
}
main()