Improve error handling

idb-keyval reported IDBTransaction.error if a `set` operation failed.  However, based on my reading of the IndexedDB spec, and based on testing, IDBTransaction.error may not be set at the time the `error` event fires. This can be seen under the following scenarios:

- If IDBObjectStore.add fails because a key already exists
- If a quota is exceeded in Safari
- If the transaction is aborted immediately after an IndexedDB operation is requested

The `error` event's EventTarget should give the specific IDBRequest that failed, and that IDBRequest's error property should always be populated, so accessing `event.target` should fix this issue.

I tried using a similar approach for `abort` events, but it did not work; strangely, the abort event's target appears to be the IDBOpenRequest associated with the transaction, rather than the transaction itself, so it does not have a usable error property.

This PR also adds limited unit tests to cover this scenario. Aborting a transaction immediately after issuing a database request appears to be a good way to force an error.

Fixes jakearchibald#163

src/index.ts

@@ -5,7 +5,9 @@ export function promisifyRequest<T = undefined>(
     // @ts-ignore - file size hacks
     request.oncomplete = request.onsuccess = () => resolve(request.result);
     // @ts-ignore - file size hacks
-    request.onabort = request.onerror = () => reject(request.error);
+    request.onerror = (e) => reject((e.target as IDBRequest<T> | IDBTransaction).error);
+    // @ts-ignore - file size hacks
+    request.onabort = () => reject(request.error);
   });
 }
 

test/index.ts

@@ -13,7 +13,8 @@ import {
   setMany,
   update,
   getMany,
-  delMany
+  delMany,
+  UseStore
 } from '../src';
 import { assert as typeAssert, IsExact } from 'conditional-type-checks';
 
@@ -23,6 +24,11 @@ mocha.setup('tdd');
 (async () => {
   await promisifyRequest(indexedDB.deleteDatabase('keyval-store'));
   const customStore = createStore('custom-db', 'custom-kv');
+  const errorStore: UseStore = (txMode, callback) => customStore(txMode, (store) => {
+    const result = callback(store);
+    store.transaction.abort();
+    return result;
+  })
 
   suite('The basics', () => {
     test('get & set', async () => {
@@ -152,6 +158,28 @@ mocha.setup('tdd');
           'Error is correct type',
         );
       }
+
+      try {
+        await get('foo', errorStore);
+        assert.fail('Expected throw');
+      } catch (err) {
+        assert.strictEqual(
+          (err as DOMException).name,
+          'AbortError',
+          'Error is correct type',
+        );
+      }
+
+      try {
+        await set('foo', 'bar', errorStore);
+        assert.fail('Expected throw');
+      } catch (err) {
+        assert.strictEqual(
+          (err as DOMException).name,
+          'AbortError',
+          'Error is correct type',
+        );
+      }
     });
   });