Node.js wait for callback of REST service that makes HTTP request

A common practice is to use the async module.

npm install async

The async module has primitives to handle various forms of asynchronous events.

In your case, the async#parallel call will allow you to make requests to all external APIs at the same time and then combine the results for return to the requester.

Since you’re making external http requests, you will probably find the request module helpful as well.

npm install request

Using request and async#parallel your route handler would look something like this…

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

exports.handler = function(req, res) {
  async.parallel([
    /*
     * First external endpoint
     */
    function(callback) {
      var url = "http://external1.com/api/some_endpoint";
      request(url, function(err, response, body) {
        // JSON body
        if(err) { console.log(err); callback(true); return; }
        obj = JSON.parse(body);
        callback(false, obj);
      });
    },
    /*
     * Second external endpoint
     */
    function(callback) {
      var url = "http://external2.com/api/some_endpoint";
      request(url, function(err, response, body) {
        // JSON body
        if(err) { console.log(err); callback(true); return; }
        obj = JSON.parse(body);
        callback(false, obj);
      });
    },
  ],
  /*
   * Collate results
   */
  function(err, results) {
    if(err) { console.log(err); res.send(500,"Server Error"); return; }
    res.send({api1:results[0], api2:results[1]});
  }
  );
};

You can also read about other callback sequencing methods here.