Question

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How to run multiple asynchronous functions then execute callbacks?

rated 0 times [ 38] [ 5] / answers: 1 / hits: 46138 / 9 Years ago, mon, september 28, 2015, 12:00:00

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?

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maxinec

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3 Years ago

answered 9 Years ago turnerf · Accepted Answer

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

});