Added tests

README.md

@@ -51,6 +51,12 @@ Provide a custom abort reason:
 promise.abort(new Error('Operation aborted'));
 ```
 
+Abort promise if an external signal is aborted:
+
+```ts
+promise.withSignal(signal);
+```
+
 # `Deferred`
 
 The promise that can be resolved externally.

src/main/AbortablePromise.ts

@@ -63,9 +63,13 @@ export class AbortablePromise<T> extends Promise<T> {
    * @returns This promise.
    */
   withSignal(signal: AbortSignal): this {
-    signal.addEventListener('abort', () => {
+    if (signal.aborted) {
       this.abort(signal.reason);
-    });
+    } else {
+      signal.addEventListener('abort', () => {
+        this.abort(signal.reason);
+      });
+    }
     return this;
   }
 }

src/test/AbortablePromise.test.ts

@@ -51,4 +51,48 @@ describe('AbortablePromise', () => {
     await expect(promise).resolves.toBe(111);
     expect(promise['_abortController'].signal.aborted).toBe(true);
   });
+
+  test('aborts if signal is aborted asynchronously', async () => {
+    const promise = new AbortablePromise(() => {});
+    const abortController = new AbortController();
+
+    promise.withSignal(abortController.signal);
+
+    abortController.abort(111);
+
+    await expect(promise).rejects.toBe(abortController.signal.reason);
+  });
+
+  test('does not abort if an external signal is not aborted', async () => {
+    const promise = new AbortablePromise(resolve => {
+      resolve(111);
+    });
+    const abortController = new AbortController();
+
+    promise.withSignal(abortController.signal);
+
+    await expect(promise).resolves.toBe(111);
+  });
+
+  test('aborts if an external signal is aborted asynchronously', async () => {
+    const promise = new AbortablePromise(() => {});
+    const abortController = new AbortController();
+
+    promise.withSignal(abortController.signal);
+
+    abortController.abort(111);
+
+    await expect(promise).rejects.toBe(abortController.signal.reason);
+  });
+
+  test('aborts if an external signal is aborted before subscription', async () => {
+    const promise = new AbortablePromise(() => {});
+    const abortController = new AbortController();
+
+    abortController.abort(111);
+
+    promise.withSignal(abortController.signal);
+
+    await expect(promise).rejects.toBe(abortController.signal.reason);
+  });
 });