How to get more than one independent response data in Express js app.get callback

Question

What is the best practice to send two independed MongoDB results in Express application via HTTP Method?

Here is a short example which makes it clear:

//app.js
var express = require('express');
var app = express();
var testController = require('./controllers/test');
app.get('/test', testController.getCounts);
...

Following getCounts() function wouldn't work because I can't send the response twice.

///controllers/test
exports.getCounts = function(req,res) {
   Object1.count({},function(err,count){
    res.send({count:count});
   });
   Object2.count({},function(err,count){
    res.send({count:count});
   });
};

Anyway, I would like to have those two counts in one response object.

Should I call Object2.count in the callback of Object1 even if they are not dependent to each other?

Or should I re-design it somehow else?

Thank you!

Answer 1

You should use Promise to achieve this task :

 function getCount(obj) {
    return new Promise(function (resolve, reject) {
        obj.count({}, function(err,count) {
             if(err) reject();
             else resolve(count);
        });
    });
 }

With Promise.all you can trigger the two request and retrieve the results in order to add it to the response

 exports.getCounts = function(req,res) {
    Promise.all([getCount(Object1), getCount(Object2)])
    .then(function success(result) {
        res.send({'count1':result[0], 'count2':result[1]});
    });
 });

Answer 2

When you call res.send you will end the response for the request. You could instead use res.write , which will send a chunk to the client, and when done call res.end ;

Example:

app.get('/endpoint', function(req, res) {
   res.write('Hello');
   res.write('World');
   res.end();
});

However, it seems like you are trying to send json back to the client which raises and problem: writing to object separately will not be valid json.

Example:

app.get('/endpoint', function(req, res) {
   res.write({foo:'bar'});
   res.write({hello:'world'});
   res.end();
});

The response body will now be: {foo:'bar'}{hello:'world'} which is not valid json.

There will also be a race condition between the two db queries, which means that you are not certain about the order of the data in the response.

Suggestion:

exports.getCounts = function(req,res) {
  var output = {};      

  Object1.count({},function(err,count){
     output.count1 = count;

     Object2.count({},function(err,count){
       output.count2 = count;
       res.send(output);
     });
  });
};

//Response body
{
   count1: [value],
   count2: [value]
}