a little example of what you'd need to to to create and resolve a promise off the main thread in Objective-C++

offmain-promise-res.mm

#include <napi.h>

Napi::Value NoOp(const Napi::CallbackInfo &info) {
  return info.Env().Undefined();
}

Napi::Promise DoAMacOSNativeThing(const Napi::CallbackInfo &info) {
  Napi::Env env = info.Env();
  Napi::Promise::Deferred deferred = Napi::Promise::Deferred::New(env);
  Napi::ThreadSafeFunction ts_fn =
      Napi::ThreadSafeFunction::New(env, Napi::Function::New(env, NoOp),
                                    "myCallback", 0, 1, [](Napi::Env) {});

  [doANativeMacOSThingWithCompletionBlock completion:^(Data blockData,
                                                       NSError *error) {
    // we don't care about noop_cb, we'll never call it (it's what we passed as
    // Napi::Function::New(env, NoOp) above)
    auto callback = [=](Napi::Env env, Napi::Function noop_cb, DataType *data) {
      deferred.Resolve(/* convert your data to a Napi::Value here */);
    };
    // DataType must be a ptr, so convert blockData to determine what you want
    // to resolve here or pass its address
    ts_fn.BlockingCall(&blockData, callback);
  }];

  return deferred.Promise();
}

Napi::Object Init(Napi::Env env, Napi::Object exports) {
  exports.Set(Napi::String::New(env, "doAMacOSNativeThing"),
              Napi::Function::New(env, DoAMacOSNativeThing));
  return exports;
}

NODE_API_MODULE(target_name, Init)