node.js: how to implement a routing method with an async call inside a forEach loop?

Question

I have to implement a logic like this:

var express = require('express'),
    request = require('request');

var array = [
  { url: 'http://www.example1.com' },
  { url: 'http://www.example2.com' },
];

express.Router().get('/job', function (req, res) {
  results = [];
  array.forEach(function (item) {
    request(item.url, function (error, response, body) {
      if (!error && response.statusCode == 200) {
        results.push(body);
      }
  });
  res.json(results); // <== wrong, results array is empty, here...
});

How do I call res.json(results) being sure forEach loop is ended?

UPDATE: it is not a duplicate of suggested answer!

I need to res.json() when all requests are completed, and not when forEach loop is finished... :-(

possible duplicate of [Callback after ForEach is done.](http://stackoverflow.com/questions/18983138/callback-after-foreach-is-done) — Chris Thompson, Sep 15 '15 at 16:14
`forEach` _is_ done. It's the asynchronous `request()` function that isn't. — Chris Thompson, Sep 15 '15 at 16:15

score 3 · Accepted Answer · answered Sep 15 '15 at 16:18

A simple way to accomplish this:

var express = require('express'),
    request = require('request');

var array = [
  { url: 'http://www.example1.com' },
  { url: 'http://www.example2.com' },
];

//store number of completed calls
var completed = 0;

express.Router().get('/job', function (req, res) {
  results = [];
  array.forEach(function (item) {
    request(item.url, function (error, response, body) {
      if (!error && response.statusCode == 200) {
        results.push(body);

        completed = completed + 1; //iterate counter
        onComplete(); //attempt completion
      }
  });

  function onComplete(){
    if(completed != array.length) {
      return;
    }
    res.json(results); 
    // ... rest of code that relies on results
  };

});

Alexandr Lazarev · Answer 2 · 2015-09-15T16:31:32.583

2

You can use asnyc series to achieve this. Something like this:

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

...

async.series([
    function(callback) {
        request('http://www.example1.com', function (error, response, body) {
            if (!error && response.statusCode == 200) {
                callback(null, body);
            }
        });
    },
    function(callback) {
        request('http://www.example2.com', function (error, response, body) {
            if (!error && response.statusCode == 200) {
                callback(null, body);
            }
        });
    }
], function(err, res) {
    res.json(res);
});

edited Sep 15 '15 at 16:31

answered Sep 15 '15 at 16:16

Alexandr Lazarev

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In real life, I don't have 2 requests, but thousands... :-( – MarcoS Sep 15 '15 at 17:42
1

I don't think then, that it is a good idea to wait for thousands of request to finish. Maybe you should change your logic and look towards a background process for handeling them? – Alexandr Lazarev Sep 15 '15 at 17:47
Good point! Some direction where I can find some info about running node as a background process? – MarcoS Sep 15 '15 at 17:52
1

Here is a good article: http://www.andygup.net/node-js-moving-intensive-tasks-to-a-child-process/ – Alexandr Lazarev Sep 16 '15 at 08:57
Thanks... I think I will not adopt suggested approach: I will just run a node process as a cron job; the process state will be shared on the database... – MarcoS Sep 16 '15 at 09:02
Yes, it`s definitely a soultion. – Alexandr Lazarev Sep 16 '15 at 09:06

score 1 · Answer 3 · answered Sep 15 '15 at 16:56

You should use promises! (This example works with last version of node because supports native promises)

var arrayPromises = [];

array.forEach(function (item) {

   arrayPromises.push(new Promise(function(resolve, reject) {
       request(item.url, function (error, response, body) {
          if (!error && response.statusCode == 200) {
             resolve(body);
          }else{
            resolve();
          }
       });
   }));
});

Promise.all(arrayPromises).then(function(results){
  res.json(results);
});

node.js: how to implement a routing method with an async call inside a forEach loop?

3 Answers3