Update beginners-guide.md

Added simple example how to use bluebird. I faced issue finding basic code to implement bluebird. So I added few lines of code for other people to have an easy way to begin with.

docs/docs/beginners-guide.md

@@ -4,3 +4,62 @@ title: Beginner's Guide
 ---
 
 [beginners-guide](unfinished-article)
+
+proper implementation of bluebird will help you chain your function synchronously.
+
+Example 1 :
+
+Code :
+
+ var express = require('express');
+ var fs = require('fs');
+ var app = express();
+
+
+ app.get('/index', function(req,res){
+ var index =fs.readFile('./views/index.html', 'utf8') //Async function
+ res.send(index)
+ });
+ var port = process.env.PORT || 3000
+ app.listen(port, function () {
+ console.log("Server is running at:" + port)
+ })
+
+In above code, it is suppose to fetch the index.html file and then send the response. But when you run it, it will not return you anything but an empty page.
+Reason : fs.readFile is an async methods, so even before read operation is completed action is passed to next command which is res.send.
+ So empty response is sent.
+
+## Solution with bluebird
+
+code :
+
+ var express = require('express');
+ var promise = require('bluebird');
+ var fs = require('fs');
+ var app = express();
+ var index;
+ var getIndex = function(){ 
+ return new promise(function(resolve,reject){
+
+ fs.readFile('./views/index.html', 'utf8', function(err,data){
+ if(err){
+ reject(err);
+ }
+ else{
+ index=data;
+ resolve(index);
+ }
+ })
+ })
+ };
+
+ app.get('/index', function(req,res){
+ getIndex().then( function(){ //bluebird promise objects make sures that next command is executed only once promise is either resolved or rejected.
+ res.send(index)
+ });
+ })
+
+ var port = process.env.PORT || 3000
+ app.listen(port, function () {
+ console.log("Server is running at:" + port)
+ })