Event Emitter Class in JavaScript

EventEmitter.js

/*
 * Create an event emitter that goes like this
 * emitter = new Emitter();
 *
 * Allows you to subscribe to some event
 * sub1 = emitter.subscribe('function_name', callback1);
 * (you can have multiple callbacks to the same event)
 * sub2 = emitter.subscribe('function_name', callback2);
 *
 * You can emit the event you want with this api
 * (you can receive 'n' number of arguments)
 * sub1.emit('function_name', foo, bar);
 *
 * And allows you to release the subscription like this
 * (but you should be able to still emit from sub2)
 * sub1.release();
 */
 
class Emitter {

  // Keeps track of subscriptions as <name: string, callbacks: []>.
  subscriptions = new Map();

  constuctor() {}
  
  /**
   * Creates a new subscription or adds the callback to an
   * existing subscription.
   */
  subscribe = (functionName, callback) => {
    const subscription = this.subscriptions.get(functionName);
    
    if (subscription) {
      subscription.push(callback);
    } else {
      subscriptions.set(functionName, [callback]);
    }
    
    return this;
  };
  
  /**
   * When subscription.emit() is called, all of the callbacks
   * are executed for that subscription.
   */ 
  emit = (functionName, ...rest) => {
    const subscription = this.subscriptions.get(functionName);
    
    if (subscription) {
      subscription.forEach(cb => cb(...rest));
    }
  };
  
  /**
   * subscription.release() removes the callback for
   * that event and updates the subscription.
   */ 
  release = (functionName, callback) => {
     const subscription = this.subscriptions.get(functionName);
    
    if (subscription) {
      const updatedSubscription = subscription.filter(cb => cb !== callback);
      subscriptions.set(functionName, updatedSubscription);
    }
  };
}