4 files changed, 15 insertions, 10 deletions
diff --git a/core/libdeno/api.cc b/core/libdeno/api.cc
index 8a3a56156..30f82b6cc 100644
--- a/core/libdeno/api.cc
+++ b/core/libdeno/api.cc
@@ -153,11 +153,15 @@ void deno_pinned_buf_delete(deno_pinned_buf* buf) {
   auto _ = deno::PinnedBuf(buf);
 }
 
-void deno_respond(Deno* d_, void* user_data, deno_buf buf) {
+void deno_respond(Deno* d_, void* user_data, deno_buf buf, int* promise_id) {
   auto* d = unwrap(d_);
   if (d->current_args_ != nullptr) {
     // Synchronous response.
-    if (buf.data_ptr != nullptr) {
+    if (promise_id != nullptr) {
+      auto number = v8::Number::New(d->isolate_, *promise_id);
+      d->current_args_->GetReturnValue().Set(number);
+    } else {
+      CHECK_NOT_NULL(buf.data_ptr);
       auto ab = deno::ImportBuf(d, buf);
       d->current_args_->GetReturnValue().Set(ab);
     }
diff --git a/core/libdeno/deno.h b/core/libdeno/deno.h
index 745285554..4f29f2c7a 100644
--- a/core/libdeno/deno.h
+++ b/core/libdeno/deno.h
@@ -81,8 +81,10 @@ void deno_execute(Deno* d, void* user_data, const char* js_filename,
 // deno_respond sends up to one message back for every deno_recv_cb made.
 //
 // If this is called during deno_recv_cb, the issuing libdeno.send() in
-// javascript will synchronously return the specified buf as an ArrayBuffer (or
-// null if buf is empty).
+// javascript will synchronously return the specified promise_id(number)
+// or buf(Uint8Array) (or null if buf and promise_id are both null/empty).
+// Calling with non-null for both buf and promise_id will result in the
+// promise_id being returned.
 //
 // If this is called after deno_recv_cb has returned, the deno_respond
 // will call into the JS callback specified by libdeno.recv().
@@ -92,7 +94,7 @@ void deno_execute(Deno* d, void* user_data, const char* js_filename,
 // releasing its memory.)
 //
 // If a JS exception was encountered, deno_last_exception() will be non-NULL.
-void deno_respond(Deno* d, void* user_data, deno_buf buf);
+void deno_respond(Deno* d, void* user_data, deno_buf buf, int* promise_id);
 
 // consumes zero_copy
 void deno_pinned_buf_delete(deno_pinned_buf* buf);
diff --git a/core/libdeno/libdeno.d.ts b/core/libdeno/libdeno.d.ts
index 1bc7367d9..093e846ab 100644
--- a/core/libdeno/libdeno.d.ts
+++ b/core/libdeno/libdeno.d.ts
@@ -12,14 +12,13 @@ interface EvalErrorInfo {
   thrown: any;
 }
 
-declare interface MessageCallback {
-  (msg: Uint8Array): void;
-}
+declare type MessageCallbackInternal = (msg: Uint8Array) => void;
 
 declare interface DenoCore {
-  recv(cb: MessageCallback): void;
+  recv(cb: MessageCallbackInternal): void;
 
   send(
+    cmdId: number,
     control: null | ArrayBufferView,
     data?: ArrayBufferView
   ): null | Uint8Array;
diff --git a/core/libdeno/libdeno_test.cc b/core/libdeno/libdeno_test.cc
index 485c95bff..b72a8e098 100644
--- a/core/libdeno/libdeno_test.cc
+++ b/core/libdeno/libdeno_test.cc
@@ -75,7 +75,7 @@ TEST(LibDenoTest, RecvReturnBar) {
     EXPECT_EQ(buf.data_ptr[1], 'b');
     EXPECT_EQ(buf.data_ptr[2], 'c');
     uint8_t response[] = {'b', 'a', 'r'};
-    deno_respond(d, user_data, {response, sizeof response});
+    deno_respond(d, user_data, {response, sizeof response}, nullptr);
   };
   Deno* d = deno_new(deno_config{0, snapshot, empty, recv_cb, nullptr});
   deno_execute(d, d, "a.js", "RecvReturnBar()");