Example of simple implementation of Javascript Promise mechanism in nodejs_node.js

var EventEmitter = require('events').EventEmitter;
var http = require('http');
var util = require('util');
//Define promise object
var Promise = function(){
// Implement inherited event class
EventEmitter.call(this);
}
//Inherit event general method
util.inherits(Promise, EventEmitter);
//The then method is the method in the promise/A specification
Promise.prototype.then = function(successHandler, errorHandler, progressHandler){
If (typeof successHandler == 'function'){
This.once('success', successHandler);
}
If (typeof errorHandler === 'function'){
This.once('error', errorHandler);
}
If (typeof progressHandler === 'function'){
This.on('process', progressHandler);
}
Return this;
}

//Define deferred object
//Contains a state and a promise object
var Deferred = function(){
This.state = 'unfulfilled';
This.promise = new Promise();
}
Deferred.prototype.resolve = function(obj){
This.state = 'fulfilled';
This.promise.emit('success', obj);
}
Deferred.prototype.reject = function(err){
This.state = 'failed';
This.promise.emit('error', err);
}
Deferred.prototype.progress = function(data){
This.promise.emit('process', data);
}

//Use an http request to apply the promise/deferred defined above

var client = function(){
var options = {
Hostname:'www.baidu.com',
Port:80,
        path:'/',
Method: 'get'
};
var deferred = new Deferred();
var req = http.request(options, function(res){
          res.setEncoding('utf-8');
      var data = '';
          res.on('data', function(chunk){
              data = chunk;
               deferred.progress(chunk);
        });
          res.on('end', function(){
              deferred.resolve(data);
        });
});
req.on('error', function(err){
         deferred.reject(err);
})
req.end();
Return deferred.promise;
}
client().then(function(data){
console.log('Request completed', data);
}, function(err){
Console.log('Access error', err);
}, function(chunk){
console.log('reading', chunk);
});