How to code with Promise from async.js?

Question

Background

I usually write node.js script based on async.js to control the work flow. Sometimes I found that based on async.js, the code seems still a 'hell'. With multiple nest the code is not readable and tricky to maintain. I did some search here and found some useful resources - but most of them are general concepts. So I am going to ask a question. Any feedback would be appreciated.

My common code

var request = require('request');
var async = require('async');

var array = [1, 2, 3, 4 ,5 ,6];
var url = 'http://www.google.com';
async.mapLimit(array, 3, function(number, callback) {
       request(url + number, function(error, res, body){
                  //do sth
                 callback();
       });
}, function(err){//do nothing});

Question

So this time I want to replace this kind of code by Promise. Could you please help? If the question is duplicated, please suggest some useful resource url. Thanks!

Answer 1

You will need to Promisify request, which can be done with native promises, but since you're using NodeJS, it would be preferable if you used the Bluebird promise library, which is almost strictly superior to the native implementation.

Then you can do:

const Promise = require('bluebird');
const requestAsync = Promise.promisify(request); // This is what bluebird saves you
// requestAsync returns a promise instead of accepting a callback.

const array = [1, 2, 3, 4, 5, 6];

const requestSingle = number => requestAsync(`${url}${number}`);

Promise.map(array, requestSingle, {concurrency: 3});

Note that Promise.map() is a Bluebird functionality, and not native.

The closest you can easily get with native is:

const arrayOfRequestPromises = array.map(requestSingle);

Promise.all(arrayOfRequestPromises)
  .then(allResults => {
    // do something
  })
  .catch(err => {
    // handle errors
  });

But then you lose the maximum 3 concurrent requests option.