refactor(eventemmiter.ts): implement Subscriptions class by extending…

… Map

Implement Subscriptions class by Extending Map to get a type-safe get() method and simplify the code
of EventEmitter.

src/EventEmitter.ts

@@ -1,15 +1,21 @@
 export type EventCallback<T = void> = (...args: Array<any>) => T;
-export type Subscriptions = Map<
+
+export class Subscriptions extends Map<
   string,
   Array<{ id: symbol; callback: EventCallback }>
->;
+> {
+  get(key: string): { id: symbol; callback: EventCallback }[] {
+    return super.get(key) || [];
+  }
+}
+
 export type Unsubscribe = () => void;
 
 export class EventEmitter {
   subscriptions: Subscriptions;
 
   constructor() {
-    this.subscriptions = new Map();
+    this.subscriptions = new Subscriptions();
   }
 
   /**
@@ -19,19 +25,17 @@ export class EventEmitter {
    */
   subscribe(eventName: string, callback: EventCallback): Unsubscribe {
     const id = Symbol(callback.toString());
-    this.subscriptions.set(
-      eventName,
-      this.subscriptions.has(eventName)
-        ? [...(this.subscriptions.get(eventName) || []), { id, callback }]
-        : [{ id, callback }]
-    );
+    this.subscriptions.set(eventName, [
+      ...this.subscriptions.get(eventName),
+      { id, callback },
+    ]);
 
     return () =>
       this.subscriptions.set(
         eventName,
-        (this.subscriptions.get(eventName) || []).filter(
-          ({ id: subscriptionId }) => subscriptionId !== id
-        )
+        this.subscriptions
+          .get(eventName)
+          .filter(({ id: subscriptionId }) => subscriptionId !== id)
       );
   }
 
@@ -40,8 +44,8 @@ export class EventEmitter {
    * @param {Array<any>} args
    */
   emit(eventName: string, ...args: Array<any>): void {
-    (this.subscriptions.get(eventName) || []).forEach(({ callback }) =>
-      callback(...args)
-    );
+    this.subscriptions
+      .get(eventName)
+      .forEach(({ callback }) => callback(...args));
   }
 }