javascript - 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
});