An example of a simple implementation of the Javascript Promise mechanism in nodejs

Promise /deferred is A good specification for handling asynchronous call coding. The following nodejs code is used as A class to implement A simple implementation of the promise/A specification

/**

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 * User: xuwenmin

 * Date: 14-4-1

 * Time:  In the morning 9:54

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 */
var EventEmitter = require('events').EventEmitter;

var http = require('http');

var util = require('util');

//Define a promise object 

var Promise = function(){

    //Implement the inherited event class 

    EventEmitter.call(this);

}

//Inherited 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 the delay 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 is completed ', data);

}, function(err){

    console.log(' Access errors ', err);

}, function(chunk){

    console.log(' Is being read ', chunk);

});

Save the code as promise.js, you can run under the command line, directly enter node promise.js, you can see the effect.