javascript继承扩展类方法实现

javascript没有原生的继承语法，这确实很让人困惑，但是广大人民群从的智慧是无穷的。最近呢，正尝到一点从源码中学习的甜头，不分享一下实在难以平复激动的心情。前不久改造视频播放插件的时候，找到了videojs这个优秀的开源项目。经过一段时间深入学习它的源码，发现它的继承机制写的很好，而且短小精悍，于是决定把它拔离出来，做成一个单独的库，方便以后项目中使用。

//定义一个命名空间
var xut = {};

/**
 * Core Object/Class for objects that use inheritance + contstructors
 *
 * @class
 * @constructor
 */
xut.CoreObject = xut['CoreObject'] = function(){};

/**
 * Create a new object that inherits from this Object
 *
 *     var Animal = CoreObject.extend();
 *     var Horse = Animal.extend();
 *
 * @param {Object} props Functions and properties to be applied to the
 *                       new object's prototype
 * @return {xut.CoreObject} An object that inherits from CoreObject
 * @this {*}
 */
xut.CoreObject.extend = function(props){
  var init, subObj;

  props = props || {};
  // Set up the constructor using the supplied init method
  // or using the init of the parent object
  // Make sure to check the unobfuscated version for external libs
  init = props['init'] || props.init || this.prototype['init'] || this.prototype.init || function(){};
  // In Resig's simple class inheritance (previously used) the constructor
  //  is a function that calls `this.init.apply(arguments)`
  // However that would prevent us from using `ParentObject.call(this);`
  //  in a Child constuctor because the `this` in `this.init`
  //  would still refer to the Child and cause an inifinite loop.
  // We would instead have to do
  //    `ParentObject.prototype.init.apply(this, argumnents);`
  //  Bleh. We're not creating a _super() function, so it's good to keep
  //  the parent constructor reference simple.
  subObj = function(){
    init.apply(this, arguments);
  };

  // Inherit from this object's prototype
  subObj.prototype = xut.obj.create(this.prototype);
  // Reset the constructor property for subObj otherwise
  // instances of subObj would have the constructor of the parent Object
  subObj.prototype.constructor = subObj;

  // Make the class extendable
  subObj.extend = xut.CoreObject.extend;
  // Make a function for creating instances
  subObj.create = xut.CoreObject.create;

  // Extend subObj's prototype with functions and other properties from props
  for (var name in props) {
    if (props.hasOwnProperty(name)) {
      subObj.prototype[name] = props[name];
    }
  }

  return subObj;
};

xut.obj = {};

xut.obj.create = Object.create || function(obj){
  //Create a new function called 'F' which is just an empty object.
  function F() {}

  //the prototype of the 'F' function should point to the
  //parameter of the anonymous function.
  F.prototype = obj;

  //create a new constructor function based off of the 'F' function.
  return new F();
};

/**
 * Create a new instace of this Object class
 *
 *     var myAnimal = Animal.create();
 *
 * @return {xut.CoreObject} An instance of a CoreObject subclass
 * @this {*}
 */
xut.CoreObject.create = function(){
  // Create a new object that inherits from this object's prototype
  var inst = xut.obj.create(this.prototype);

  // Apply this constructor function to the new object
  this.apply(inst, arguments);

  // Return the new object
  return inst;
}; 

项目地址:https://github.com/bjtqti/mini-extend

关于它的使用，我会抽空在git上详细说明，先作一个简单的案例，有兴趣的读者可以自行分析一下它的妙处：

var Animal = xut.CoreObject.extend({
    init : function(name){
        this.name = name;
    }
});

Animal.prototype.getName = function(){
    return this.name;
}

Animal.prototype.makeSound = function(){
    alert('xxx');
}

var Bird = Animal.extend();

var Dog = Animal.extend({
    makeSound : function(){
        alert('wang...')
    }
});

var b = new Bird('bage');
var d = new Dog('wang');
d.makeSound()

b.makeSound()

子类可以很方便的继承父类的功能，而且子类也可以很自由的修改父类的功能，还不会影响它子类。这对于熟悉php/java等语言的程序员来说，这简直就是废话，但是js用这一百来行的代码实现，做到这种程度，非常的不容易。