Async await in native module

Question

I'm learning how to use node-addon-api and now I'm stuck on async/await management. I cannot understand how to handle the case where a native function receive an object which has an async function.

Javascript:

const addon = require('bindings')('addon');

class Foo {
    async doAsync() {
        ...
    }
}

const useFoo = () => {
    const foo = new Foo();
    await addon.doStuff(foo);
}

Native:

#include <napi.h>
using namespace Napi;

Napi::Object doStuff(const Napi::CallbackInfo& info) {
     Napi::Env env = info.Env();
     MyObject* foo = Napi::ObjectWrap<MyObject>::Unwrap(info[0].As<Napi::Object>());
     // How should I await for this?
     foo.doAsync();
     ...
}

Napi::Object InitAll(Napi::Env env, Napi::Object exports) {
    exports.Set(Napi::String::New(env, "doStuff"), Napi::Function::New(env, doStuff));
    return exports;
}

NODE_API_MODULE(addon, InitAll)

My problem is that I do not find any documentation about this behavior. I read about how to create a promise in native but not how to use it when received from JS.

What do you want to do with the promise? Probably you'll want to call its `then` method, there's not much else. (And no, I doubt there's an `await` counterpart in C++) — Bergi, Jul 07 '19 at 12:33
Yes, I would use a JS class which has async methods in native code. I do not understand how this kind of object should be wrapped. — Marco Stramezzi, Jul 08 '19 at 06:33
You wouldn't wrap it differently than a normal method returning a promise. — Bergi, Jul 08 '19 at 09:49
There will be async/await in C++ 20, called coroutine, and you can use it as an experimental feature of many compilers. Then, create your own awaitable wrapper around Promise. — fjch1997, Dec 11 '19 at 20:18
To await something is equally to poll for a flag or to use a callback - in the Main thread it means to pause the execution - thus you'd wrap this in a std::thread or similar and return a deferred:: promise() - but now I wonder what information is so critical that it has to be called in native and cannot be awaited in js before ? — Estradiaz, Feb 26 '20 at 07:40

Jordan Snyder · Answer 1 · 2020-02-26T17:03:51.300

On the C++ addon side, you have to implement functionality asynchronously and return a Promise. See the link below for an example of exactly that.

On the Javascript side, you have to wrap that async/Promise call in an explicitly async Javascript function. Otherwise, if you try to await a Promise, you'll get an error about the function not being asynchronous. So on your JS side, something like the following should work:

async function init() {
  let obj = new YourAddon()
  let cnxnResult = await obj.connect()
}
init()

So you could create your own Javascript class (like your code examples) to wrap this C++ functionality and keep everything async/await friendly.

I found this link to be particularly helpful for showing how the C++ addon side is architected: Promise / AsyncWorker example

Async await in native module

1 Answers1