Using Async Promises in native node.js Addons

Small example to demonstrate how to implement promises asynchronously in node.js native c++ addon modules. This addon uses nan and is targeted at node 8.

Example

This addon creates a Person object that has two methods, say (which returns a Promise) and also a sayCallback that has the same functionality but implemented as a callback for comparison.

const person = require('./build/Release/person.node')
const bob = person.create('bob')

async function sayItAsyncAwait () {
  let result

  try {
    result = await bob.say('hello async/await')
  } catch (err) {
    console.error(err)
    throw err
  }

  console.log(result)
}

sayItAsyncAwait()

function sayItPromise () {
  bob.say('hello promise').then(result => {
    console.log(result)
  }).catch(err => {
    console.error(err)
  })
}

sayItPromise()

function sayItCallback () {
  bob.sayCallback('hello callback', function (err, result) {
    if (err) {
      console.error(err)
    } else {
      console.log(result)
    }
  })
}

sayItCallback()

How it works

In short, SayWorker.cc is still a Nan::AsyncWorker (as you would do with a regular callback), but instead of being passed a callback, a v8::promise is passed instead. This is then resolved in the HandleOKCallback.

N-API

N-API is the successor to NaN for creating native node modules, and has first class support for promises: https://nodejs.org/api/n-api.html#n_api_promises