Add async JSPromise.result property

IntegrationTests/TestSuites/Sources/ConcurrencyTests/main.swift

@@ -32,6 +32,7 @@ func entrypoint() async throws {
             resolve(.success(1))
         })
         try await expectEqual(p.value, 1)
+        try await expectEqual(p.result, .success(.number(1)))
     }
 
     try await asyncTest("await rejected Promise") {
@@ -41,6 +42,7 @@ func entrypoint() async throws {
         let error = try await expectAsyncThrow(await p.value)
         let jsValue = try expectCast(error, to: JSValue.self)
         try expectEqual(jsValue, 3)
+        try await expectEqual(p.result, .failure(.number(3)))
     }
 
     try await asyncTest("Continuation") {

Sources/JavaScriptEventLoop/JavaScriptEventLoop.swift

@@ -116,6 +116,24 @@ public extension JSPromise {
             }
         }
     }
+
+    /// Wait for the promise to complete, returning its result or exception as a Result.
+    var result: Result<JSValue, JSValue> {
+        get async {
+            await withUnsafeContinuation { [self] continuation in
+                self.then(
+                    success: {
+                        continuation.resume(returning: .success($0))
+                        return JSValue.undefined
+                    },
+                    failure: {
+                        continuation.resume(returning: .failure($0))
+                        return JSValue.undefined
+                    }
+                )
+            }
+        }
+    }
 }
 
 #endif