How to run multiple asynchronous functions then execute callbacks?

Question

In my Node.js code I need to make 2 or 3 API calls, and each will return some data. After all API calls are complete, I want to collect all the data into a single JSON object to send to the frontend.

I know how to do this using the API callbacks (the next call will happen in the previous call's callback) but this would be slow:

//1st request request('http://www.example.com', function (err1, res1, body) {      //2nd request   request('http://www.example2.com', function (err2, res2, body2) {        //combine data and do something with it    });  }); 

I know you could also do something similar and neater with promises, but I think the same concept applies where the next call won't execute until the current one has finished.

Is there a way to call all functions at the same time, but for my final block of code to wait for all API calls to complete and supply data before executing?

Answer 1

Promises give you Promise.all() (this is true for native promises as well as library ones like bluebird's).

Update: Since Node 8, you can use util.promisify() like you would with Bluebird's .promisify()

var requestAsync = util.promisify(request); // const util = require('util') var urls = ['url1', 'url2']; Promise.all(urls.map(requestAsync)).then(allData => {     // All data available here in the order of the elements in the array }); 

So what you can do (native):

function requestAsync(url) {     return new Promise(function(resolve, reject) {         request(url, function(err, res, body) {             if (err) { return reject(err); }             return resolve([res, body]);         });     }); } Promise.all([requestAsync('url1'), requestAsync('url2')])     .then(function(allData) {         // All data available here in the order it was called.     }); 

If you have bluebird, this is even simpler:

var requestAsync = Promise.promisify(request); var urls = ['url1', 'url2']; Promise.all(urls.map(requestAsync)).then(allData => {     // All data available here in the order of the elements in the array }); 

Answer 2

Sounds like async.parallel() would also do the job if you'd like to use async:

var async = require('async');  async.parallel({     one: function(parallelCb) {         request('http://www.example1.com', function (err, res, body) {             parallelCb(null, {err: err, res: res, body: body});         });     },     two: function(parallelCb) {         request('http://www.example2.com', function (err, res, body) {             parallelCb(null, {err: err, res: res, body: body});         });     },     three: function(parallelCb) {         request('http://www.example3.com', function (err, res, body) {             parallelCb(null, {err: err, res: res, body: body});         });     } }, function(err, results) {     // results will have the results of all 3     console.log(results.one);     console.log(results.two);     console.log(results.three); });