如下是对XMLHttpRequest的封装，为一个简易的工厂模式

/* AjaxHandler interface. */

var AjaxHandler = new Interface('AjaxHandler',['request','createXhrObject']);

/* SimpleHandler class. */

var SimpleHandler = function() {}; // implements AjaxHandler
SimpleHandler.prototype = {
  request: function(method,url,callback,postVars) {
    var xhr = this.createXhrObject();
    xhr.onreadystatechange = function() {
      if(xhr.readyState !== 4) return;
      (xhr.status === 200) ? 
        callback.success(xhr.responseText,xhr.responseXML) : 
        callback.failure(xhr.status);
    };
    xhr.open(method,true);
    if(method !== 'POST') postVars = null;
    xhr.send(postVars);
  },createXhrObject: function() { // Factory method.
    var methods = [
      function() { return new XMLHttpRequest(); },function() { return new ActiveXObject('Msxml2.XMLHTTP'); },function() { return new ActiveXObject('Microsoft.XMLHTTP'); }
    ];
    
    for(var i = 0,len = methods.length; i < len; i++) {
      try {
        methods[i]();
      }
      catch(e) {
        continue;
      }
      // If we reach this point,method[i] worked.
      this.createXhrObject = methods[i]; // Memoize the method.
      return methods[i];
    }
    
    // If we reach this point,none of the methods worked.
    throw new Error('SimpleHandler: Could not create an XHR object.');
  } 
};

/* Usage. */

var myHandler = new SimpleHandler();
var callback = { 
  success: function(responseText) { alert('Success: ' + responseText); },failure: function(statusCode) { alert('Failure: ' + statusCode); } 
};
myHandler.request('GET','script.PHP',callback);

如下是辅助类的实现：

// Constructor.

var Interface = function(name,methods) {
    if(arguments.length != 2) {
        throw new Error("Interface constructor called with " + arguments.length
          + "arguments,but expected exactly 2.");
    }
    
    this.name = name;
    this.methods = [];
    for(var i = 0,len = methods.length; i < len; i++) {
        if(typeof methods[i] !== 'string') {
            throw new Error("Interface constructor expects method names to be " 
              + "passed in as a string.");
        }
        this.methods.push(methods[i]);        
    }    
};    

// Static class method.

Interface.ensureImplements = function(object) {
    if(arguments.length < 2) {
        throw new Error("Function Interface.ensureImplements called with " + 
          arguments.length  + "arguments,but expected at least 2.");
    }

    for(var i = 1,len = arguments.length; i < len; i++) {
        var interface = arguments[i];
        if(interface.constructor !== Interface) {
            throw new Error("Function Interface.ensureImplements expects arguments "   
              + "two and above to be instances of Interface.");
        }
        
        for(var j = 0,methodsLen = interface.methods.length; j < methodsLen; j++) {
            var method = interface.methods[j];
            if(!object[method] || typeof object[method] !== 'function') {
                throw new Error("Function Interface.ensureImplements: object " 
                  + "does not implement the " + interface.name 
                  + " interface. Method " + method + " was not found.");
            }
        }
    } 
};