Use functional programming for self-cleaning WebSocket callbacks

src/lib/utils/websocket_utils.ts

@@ -22,13 +22,18 @@ export interface NostrFilter {
   limit?: number;
 }
 
-export async function fetchNostrEvent(filter: NostrFilter): Promise<NostrEvent> {
-  // TODO: Improve relay selection when relay management is implemented.
-  const ws = await WebSocketPool.instance.acquire("wss://thecitadel.nostr1.com");
-  const subId = crypto.randomUUID();
+type ResolveCallback<T> = (value: T | PromiseLike<T>) => void;
+type RejectCallback = (reason?: any) => void;
+type EventHandler = (ev: Event) => void;
+type EventHandlerReject = (reject: RejectCallback) => EventHandler; 
+type EventHandlerResolve<T> = (resolve: ResolveCallback<T>) => EventHandlerReject;
 
-  const res = new Promise<NostrEvent>((resolve, reject) => {
-    ws.addEventListener("message", (ev) => {
+function handleMessage(
+  ev: MessageEvent,
+  subId: string,
+  resolve: (event: NostrEvent) => void,
+  reject: (reason: any) => void
+) {
   const data = JSON.parse(ev.data);
 
   if (data[1] !== subId) {
@@ -52,12 +57,52 @@ export async function fetchNostrEvent(filter: NostrFilter): Promise<NostrEvent>
   }
 
   resolve(event);
-    });
+}
 
-    ws.addEventListener("error", (ev) => {
+function handleError(
+  ev: Event,
+  reject: (reason: any) => void
+) {
   reject(ev);
+}
+
+export async function fetchNostrEvent(filter: NostrFilter): Promise<NostrEvent> {
+  // TODO: Improve relay selection when relay management is implemented.
+  const ws = await WebSocketPool.instance.acquire("wss://thecitadel.nostr1.com");
+  const subId = crypto.randomUUID();
+
+  // AI-NOTE: Currying is used here to abstract the internal handler logic away from the WebSocket
+  // handling logic. The message and error handlers themselves can be refactored without affecting
+  // the WebSocket handling logic.
+  const curriedMessageHandler: (subId: string) => EventHandlerResolve<NostrEvent> =
+    (subId) =>
+      (resolve) =>
+        (reject) =>
+          (ev: MessageEvent) =>
+            handleMessage(ev, subId, resolve, reject);
+  const curriedErrorHandler: EventHandlerReject =
+    (reject) =>
+      (ev: Event) =>
+        handleError(ev, reject);
+
+  // AI-NOTE: These variables store references to partially-applied handlers so that the `finally`
+  // block receives the correct references to clean up the listeners.
+  let messageHandler: EventHandler;
+  let errorHandler: EventHandler;
+
+  const res = new Promise<NostrEvent>((resolve, reject) => {
+    messageHandler = curriedMessageHandler(subId)(resolve)(reject);
+    errorHandler = curriedErrorHandler(reject);
+
+    ws.addEventListener("message", messageHandler);
+    ws.addEventListener("error", errorHandler);
+  })
+  .withTimeout(2000)
+  .finally(() => {
+    ws.removeEventListener("message", messageHandler);
+    ws.removeEventListener("error", errorHandler);
+    WebSocketPool.instance.release(ws);
   });
-  }).withTimeout(2000);
 
   ws.send(JSON.stringify(["REQ", subId, filter]));
   return res;