feat(jsc): unhandled promise rejection tracker

Author: ammarahm-ed

test-app/app/src/main/assets/internal/ts_helpers.js

@@ -424,6 +424,15 @@
         handledPromise(promise);
       }
     };
+  } else if (globalThis.__engine === "JSC") {
+    // JSC's unhandled-rejection callback only fires for rejections that go
+    // unhandled (there is no "handled later" retraction event), and it is
+    // invoked with (promise, reason).
+    globalThis.onUnhandledPromiseRejectionTracker = (promise, reason) => {
+      hasBeenNotifiedProperty.set(promise, false);
+      const error = makeRejectionError(reason);
+      unhandledPromise(promise, error);
+    };
   } else if (globalThis.__engine === "Hermes") {
     HermesInternal.enablePromiseRejectionTracker({
       allRejections: true,

test-app/runtime/src/main/cpp/napi/jsc/jsr.cpp

@@ -1,4 +1,27 @@
 #include "jsr.h"
+// JSGlobalContextSetUnhandledRejectionCallback lives in this private JSC header.
+#include <JavaScriptCore/JSContextRefPrivate.h>
+
+// Native trampoline for JSC's unhandled-promise-rejection callback. JSC invokes
+// it with (promise, reason); we forward to the JS-side
+// globalThis.onUnhandledPromiseRejectionTracker (installed by ts_helpers.js),
+// mirroring how the V8 path routes rejections.
+static napi_value JscUnhandledRejectionCallback(napi_env env, napi_callback_info info) {
+    size_t argc = 2;
+    napi_value args[2];
+    napi_get_cb_info(env, info, &argc, args, nullptr, nullptr);
+
+    napi_value global, tracker;
+    napi_get_global(env, &global);
+    napi_get_named_property(env, global, "onUnhandledPromiseRejectionTracker", &tracker);
+
+    napi_valuetype type;
+    napi_typeof(env, tracker, &type);
+    if (type == napi_function) {
+        napi_call_function(env, global, tracker, argc, args, nullptr);
+    }
+    return nullptr;
+}
 
 napi_status js_create_runtime(jsr_ns_runtime *runtime) {
     if (!runtime) return napi_invalid_arg;
@@ -23,6 +46,19 @@ napi_status js_create_napi_env(napi_env* env, jsr_ns_runtime runtime) {
     napi_get_global(*env, &global);
     napi_set_named_property(*env, global, "gc", gc);
 
+    // Report unhandled promise rejections. JSC keeps the callback alive (it is
+    // stored on and marked by the global object), so no extra protection is
+    // needed. JSC only surfaces the "unhandled" event, not a later "handled"
+    // retraction, so the JS tracker treats every call as unhandled.
+    napi_value rejectionCallback;
+    napi_create_function(*env, "onUnhandledRejection", NAPI_AUTO_LENGTH,
+                         JscUnhandledRejectionCallback, nullptr, &rejectionCallback);
+    JSValueRef rejectionException = nullptr;
+    JSGlobalContextSetUnhandledRejectionCallback(
+            (*env)->context,
+            reinterpret_cast<JSObjectRef>(rejectionCallback),
+            &rejectionException);
+
 
     return napi_ok;