Node.js版Promise实现

promise.js

var EventEmitter = require('events').EventEmitter;
var util = require('util');
var fs = require('fs');

/**
 * promise 实现
 */
var Promise = function () {
    EventEmitter.call(this);
};

util.inherits(Promise, EventEmitter);

Promise.prototype.then = function (fulfilledHandler, errorHandler) {
    if (typeof fulfilledHandler === 'function') {
        this.once('success', fulfilledHandler);
    }
    if (typeof errorHandler === 'function') {
        this.once('error', errorHandler);
    }

    return this;
};


/**
 * Deferred 实现
 */
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 (obj) {
    this.state = 'failed';
    this.promise.emit('error', obj);
};

/**
 * 实例:文件读取
 */
var readFile = function  (filename, encoding) {
    var deferred = new Deferred();

    fs.readFile(filename, encoding, function (err, data) {
        if (err) {
            deferred.reject(err);
        } else {
            deferred.resolve(data);
        }
    });
    
    return deferred.promise;
};

readFile('filename', 'utf8').then(function (data) {
    console.log(this.state); // filfulled
}, function (err) {
    console.log(this.state); // failed
});