如何在Express js app.get回调中获取多个独立响应数据

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]
}