Check out our solution

One possible implementation could be

class Emitter {
  constructor() {
    /**
     * This does not make the subscriptions private
     *
     * We can look into using Private class features
     * but as of writing this solution, they are not 100% supported
     * https://caniuse.com/?search=private
     *
     * We can also look into creating getter/setter methods
     **/
    this._subscriptions = new Map();
  }

  subscribe(eventName, callback) {
    // check if eventName is already exists in our subscriptions
    if (!this._subscriptions.has(eventName)) {
      this._subscriptions.set(eventName, new Map());
    }

    // creating an unique subscription id
    const subscriptionId = Symbol();
    const eventSubscriptions = this._subscriptions.get(eventName);

    // storing the callback provided by the user
    eventSubscriptions.set(subscriptionId, callback);

    return {
      release: () =&gt; {
        // if subscriptionId is present in the eventSubscriptions
        // then release it
        if (eventSubscriptions.has(subscriptionId)) {
          eventSubscriptions.delete(subscriptionId);
        } else {
          throw new Error('This subscription has already been released');
        }
      },
    };
  }

  emit(eventName, ...args) {
    // invoking all callbacks on emit
    if (this._subscriptions.has(eventName)) {
      const eventSubscriptions = this._subscriptions.get(eventName);
      
      for (const [, callback] of eventSubscriptions) {
        callback(...args);
      }
    }
  }
}

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