Javascript实现继承的方法

  JavaScript可以有许多种方法来实现继承。

  1.原型继承

  下面的代码使用原型继承来创建一个Pet的对象，和一个继承它的Cat对象。

/**
 * Created by ztt on 2017/7/24.
 */

//Define a Pet object. Pass it a name and number of legs.
var Pet = function (name,legs) {
    this.name = name;
    this.legs = legs;
};

//Create a method that shows the Pet's name and number of legs.
Pet.prototype.getDetails = function () {
    return this.name + ' has ' + this.legs + ' legs ';
};

//Define a Cat object,inheriting from Pet.
var Cat = function (name) {
    Pet.call(this,name,4);//Call the parent object's constructor.
};

//This line performs the inheritance from Pet.
Cat.prototype.action = function () {
    return 'Catch a bird';
};

//Create an instance of Cat in petCat.
var petCat = new Cat('Felix');

var details = petCat.getDetails();  //'Felix has 4 legs'.
var action = petCat.action();       //'Catch a bird'.
petCat.name = 'Sylvester';          //Change petCat's name.
petCat.legs = 7;                    //Change petCat's number of legs!!!
details = petCat.getDetails();      //'Sylvester has 7 legs'.

  上述代码可以工作，但不是特别优雅。如果熟悉其它OOP语言，比如C++或者Java,new声明是好理解的。但是关键字prototype显得很啰嗦，并没有隐私；注意外部代码将petCat的legs属性改成了一个不合理的值：7。这种继承方法没有提供对外部继承的保护，在涉及多个程序员的复杂项目中这个缺点也许会影响很大。

  2.函数继承

  JavaScript有一个选项可以无序使用prototype或new，而是利用JavaScript的“函数继承”特性。

  

/**
 * Created by ztt on 2017/7/24.
 */

//Define a Pet object. Pass it a name and number of legs.
var pet = function (name,legs) {
    //Create an object literal (that). Include a name property for public use
    //and a getDetails() function. Legs will remain private.
    //Any local variables defined here or passed to pet as arguments will remain
    //private, but still be accessible from functions defined below.
    var that = {
        name: name,
        getDetails: function () {
            //Due to JavaScript's scoping rules, the legs variable
            //will be available in here (a closure) despite being
            //inaccessible from outside the pet object.
            return this.name + ' has ' + this.legs + ' legs ';
        }
    };
    return that;
};

//Define a Cat object,inheriting from Pet.
var Cat = function (name) {
    var that = pet(name,4); //Inherit from pet.
    //Augment cat with an action method.
    that.action = function () {
        return 'Catch a bird';
    };
    return that;
};

//Create an instance of Cat in petCat.
var petCat2 = Cat('Felix');

var details = petCat2.getDetails();  //'Felix has 4 legs'.
var action = petCat2.action();       //'Catch a bird'.
petCat2.name = 'Sylvester';          //We can change the name.
petCat2.legs = 7;                    //But not the number of legs!
details = petCat2.getDetails();      //'Sylvester has 4 legs'.

  这里没有prototype,而且所有东西都封装得很漂亮。最重要的是：legs变量是私有的。如果尝试从cat外部修改不存在的公共legs属性，仅导致创建一个没有用过的legs属性。真正的legs值安全地保存在pet的getDetails()方法创建的闭包内部。闭包在函数执行结束后，保持了函数的局部变量。在这个离职中方这个函数指的是pet()。