Solution Implementation

We're tasked with creating an EventEmitter class that can manage various event subscriptions and trigger these events when needed. Essentially, we're building the underpinnings of a custom event-handling system.

Why Use a Map for Subscriptions?

The subscriptions Property

The first thing to note is the subscriptions property, which is a JavaScript Map. A Map in JavaScript holds key-value pairs and remembers the original insertion order of the keys. This makes it an excellent choice for our subscription management because we can store the event names as keys and their respective callbacks as values.

Benefits of Using a Map

• Order Preservation: Unlike objects, the order of elements is preserved. This could be useful if the order of subscription or event triggering matters.
• Better Performance: Map operations like get, set, has, and delete are generally faster than their object counterparts.

The subscribe Method: The Heart of the EventEmitter

Step 1: Initialize the Event Subscription

When someone wants to subscribe to an event, they'll use the subscribe method, passing in the eventName and a callback function to execute when that event is emitted.

Here's the part of the code that ensures that an entry for this eventName exists in the subscriptions map:

1if (!this.subscriptions.has(eventName)) {
2    this.subscriptions.set(eventName, new Set());
3}

Why Use a Set for Callbacks?

We use a Set to store all callbacks associated with a particular eventName. The Set ensures that each callback is unique, preventing duplicate callbacks for the same event.

Step 2: Add the Callback

Once we have ensured that an entry exists for the eventName, the next step is to add the callback to this event's subscriptions:

1const subscriptions = this.subscriptions.get(eventName);
2const callbackObj = { callback };
3subscriptions.add(callbackObj);

We wrap the callback in an object (callbackObj). This gives us more flexibility for future enhancements, like adding additional metadata to the subscription.

Step 3: Return a Release Function

The method returns a release function that allows the subscriber to unsubscribe from the event:

1return {
2    release: () => {
3        subscriptions.delete(callbackObj);
4        if (subscriptions.size === 0) {
5            delete this.subscriptions.eventName;
6        }
7    },
8};

When the release function is invoked, it removes the callbackObj from the Set of subscriptions. If there are no more subscriptions for this event, it also removes the eventName entry from the subscriptions map.