JavaScript之五：继承

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五，继承

属性通过调用父类构造函数复制，方法通过原型链公用。

1> 原型链

function SuperType(){  this.property = true;}SuperType.prototype.getSuperValue = function(){  return this.property;};function SubType(){  this.subproperty = false;}SubType.prototype = new SuperType();SubType.prototype.getSubValue = function(){  return this.subproperty;}

优势：可通过instanceof和isPrototypeOf()检测

缺陷：父类的引用类型属性将被子类公用。

2> 借用构造函数

function SuperType(){  this.colors = ["red","blue","green"];}function SubType(){  SuperType.call(this);}

优势：父类的所有属性和方法都被复制到子类中

缺陷：无法公用父类的方法。

3> 组合继承

function SuperType(name){  this.name = name;  this.colors = ["red","blue","green"];}SuperType.prototype.sayName = function(){  alert(this.name);};function SubType(name, age){  SuperType.call(this, name);  this.age = age;}SubType.prototype = new SuperType();SubType.prototype.sayAge = function(){  alert(this.age);};

优势：子类从父类公用方法，复制属性。

4> 原型式继承

function object(o){  function F(){}  F.prototype = o;  return new F();}var person = {  name : "Nicholas", friends : ["Shelby", "Court", "Van"] };var anotherPerson = object(person);

5>寄生式继承

function createAnother(original){  var clone = object(original);  clone.sayHi = function(){    alert("hi");  };  return clone;}var person = { name : "Nicholas", friends : ["Shelby", "Court", "Van"]var anotherPerson = createAnother(person);

6>寄生组合式继承

function inheritPrototype(subType, superType){  var prototype = object(superType.prototype);  prototype.constructor = subType;  subType.prototype = prototype;}function SuperType(name){  this.name = name;  this.colors = ["red","blue","green"];}SuperType.prototype.sayName = function(){  alert(this.name);};function SubType(name, age){  SuperType.call(this, name);  this.age = age;}inheritPrototype(SubType, SuperType);SubType.prototype.sayAge = function(){  alert(this.age);};

最佳方案。

二，变量的作用域

JavaScript没有块级作用域。

三，垃圾回收机制

尽管JavaScript拥有垃圾回收机制，但仍然需要管理内存，原则就是将不用的全局变量设置为null，而局部变量则会在离开执行环境后自动被设为null。

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