#nodejs beginner

I came across this scenario, where I had to query the db for each item in a collection. I had to spend some time to figure out how to do this. So, I'm posting my solution here.

Since, db querying is asynchronous in nodejs, apiece of code like this would fail.

for(i=0; i<arr.length; i++) { var result = db.collection.find({_id:'some_id'}); res.write(i+':'+JSON.stringify(result)+','); } res.end();

The result would simply return undefined here since this db.collection.find() is an asynchronous call.

So, to avoid this,

for(i=0; i<arr.length; i++) { db.collection.find({_id:arr[i].id}, function(err, result) { if(!err && result.length>0){ res.write(arr[i]+':'+JSON.stringify(result[0])+','); } }); } res.end();

This will definitely solve the async call issue. But, inside res.write(), I am trying to write arr[i]. From the point the db.collection.find() call has been made to the point the result has been returned by the call, the 'i' value would have changed. So,

arr.foreach(obj) { db.collection.find({_id:obj.id}, function(err, result) { if(!err && result.length>0){ res.write(obj+':'+JSON.stringify(result[0])+','); } }); } res.end();

There is another issue here. Immediately after the foreach loop is executed, there is a res.end() statement. At this point, If there are any pending async calls to the db, the res.write() will fail after res.end() is executed. So, I tweaked the code further more.

1. Installation

Download and Install node from here

Create a project folder and navigate to the folder from command line and execute commands npm install express and npm install jade to install express.js, jade.

2. Setting up Server

Create a file named app.js inside the project folder.

Edit your app.js,

var express = require("express"); var app = express(); app.configure(function() { //Your server listens on this port app.set('port', process.env.PORT || 3001); // Setting path for your Jade Views. app.set('views', __dirname + '/views'); // Set view engine as Jade app.set('view engine', 'jade'); //use router to route request app.use(app.router); //set the path for your static files app.use(express.static(path.join(__dirname, 'public'))); //Routing Requests app.get('index', function(req, res){ //handing the request localhost:3001/index is done here res.render('index', {name:'Rocky'}); }) });