Review fixes

src/main/PubSub.ts

@@ -35,10 +35,11 @@ export class PubSub<T = void> {
   /**
    * Waits for a message that satisfies the given predicate to be published and resolves with that message.
    *
+   * @template R A subtype of T that the predicate function identifies as satisfying the condition.
    * @param predicate A function that takes the message as a parameter and returns true if the message satisfies the condition, otherwise false.
    * @returns A promise that resolves with the published message that satisfies the predicate.
    */
-  waitFor(predicate: (message: T) => boolean): Promise<T> {
+  waitFor<R extends T>(predicate: (message: T) => message is R): Promise<R> {
     return new Promise(resolve => {
       const unsubscribe = this.subscribe(message => {
         if (predicate(message)) {

src/test/PubSub.test.ts

@@ -1,4 +1,4 @@
-import { PubSub, delay } from '../main';
+import { PubSub } from '../main';
 
 describe('PubSub', () => {
   test('publishes a message', () => {
@@ -55,41 +55,15 @@ describe('PubSub', () => {
     expect(listenerMock2).toHaveBeenCalledTimes(1);
   });
 
-  test('waits for a specific message and resolves with that message', () => {
+  test('waits for a specific message and resolves with that message', async () => {
     const pubSub = new PubSub<number>();
-
-    pubSub
-      .waitFor(message => message === 42)
-      .then(message => {
-        expect(message).toBe(42);
-      });
+    const promise = pubSub.waitFor((message): message is 42 => message === 42);
 
     pubSub.publish(10);
     pubSub.publish(20);
     pubSub.publish(42);
     pubSub.publish(30);
-  });
-
-  test('does not resolve if no message matches the predicate', () => {
-    const listenerMock = jest.fn();
-    const pubSub = new PubSub<number>();
-
-    let result: number;
-
-    pubSub
-      .waitFor(message => message === 42)
-      .then(message => {
-        result = message;
-        listenerMock();
-      });
-
-    pubSub.publish(10);
-    pubSub.publish(20);
-    pubSub.publish(30);
 
-    delay(100).then(() => {
-      expect(result).toBeUndefined();
-      expect(listenerMock).toHaveBeenCalledTimes(0);
-    });
+    await expect(promise).resolves.toBe(42);
   });
 });