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Javascript Public And Private Properties And Methods

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与java等基于类的面向对象语言的private、protected、public等关键字的用途类似，基于对象的JavaScript语言，在对象构造上也存在类似的成员可见性问题。
JavaScript对象构造的可见性定义可以分为以下几种：

##私有属性（private properties）
　　
　　通过var关键字定义对象构造中变量的作用域，该变量只能在对象构造方法的作用域内被访问。如：

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function VariableTest() 
{ 
    var myVariable;//private 
} 
var vt = new VariableTest(); 
vt.myVariable;//这里会出现undefined异常

##私有方法（private methods）
　　与私有属性类似，只能在对象构造方法作用域内被访问。如：

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function MethodTest() {
    var myMethod = function() //private 
        {
            alert("private method");
        }
    this.invoke = function() {
        //能够访问到myMethod 
        myMehtod();
    }
}
var mt = new MethodTest();
mt.myMethod(); //错误。使用trycatch的话，可捕获“对象不支持此属性或方法”异常 
mt.invoke();

##公共属性（public properties）
　　有两种定义公共属性的途径：
　　（1）通过this关键字来定义。如： 

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function PrivilegedVariable() {
    this.variable = "privileged variable";
}
var pv = new PrivilegedVariable();
pv.variable; //返回 "privileged variable"

（2）通过构造方法的原型来定义。如： 

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function PublicVariable() {}
PublicVariable.prototype.variable = "public variable";
var pv = new PublicVariable();
pv.variable; //返回"public variable"

##公共方法（public methods）
　　同理，有两种定义公共方法的途径。 　　

  • 通过this关键字来定义。
  • 通过构造方法的原型来定义。
    　　这里省略。。。。。。。。。。。

##静态属性（static properties）
　　直接为对象构造方法添加的属性，不能被对象实例访问，只能供构造方法自身使用。如：

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function StaticVariable() {}
StaticVariable.variable = "static variable";
var sv = new StaticVariable();
sv.variable; //返回"undefined" 
StaticVariable.prototype.variable; //返回"undefined" 
StaticVariable.variable; //返回"static variable"

##静态方法（static methods）
　　直接为对象构造方法添加的方法，不能被对象实例访问，只能供构造方法自身使用。
　　代码省略。。。。。。。。

Example:

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function Person(n,race){ 
    this.constructor.population++;

    // ************************************************************************ 
    // PRIVATE VARIABLES AND FUNCTIONS 
    // ONLY PRIVELEGED METHODS MAY VIEW/EDIT/INVOKE 
    // *********************************************************************** 
    var alive=true, age=1;
    var maxAge=70+Math.round(Math.random()*15)+Math.round(Math.random()*15);
    function makeOlder(){ return alive = (++age <= maxAge) } 

    var myName=n?n:"John Doe";
    var weight=1;


    // ************************************************************************ 
    // PRIVILEGED METHODS 
    // MAY BE INVOKED PUBLICLY AND MAY ACCESS PRIVATE ITEMS 
    // MAY NOT BE CHANGED; MAY BE REPLACED WITH PUBLIC FLAVORS 
    // ************************************************************************ 
    this.toString=this.getName=function(){ return myName } 

    this.eat=function(){ 
        if (makeOlder()){ 
            this.dirtFactor++;
            return weight*=3;
        } else alert(myName+" can't eat, he's dead!");
    } 
    this.exercise=function(){ 
        if (makeOlder()){ 
            this.dirtFactor++;
            return weight/=2;
        } else alert(myName+" can't exercise, he's dead!");
    } 
    this.weigh=function(){ return weight } 
    this.getRace=function(){ return race } 
    this.getAge=function(){ return age } 
    this.muchTimePasses=function(){ age+=50; this.dirtFactor=10; } 


    // ************************************************************************ 
    // PUBLIC PROPERTIES -- ANYONE MAY READ/WRITE 
    // ************************************************************************ 
    this.clothing="nothing/naked";
    this.dirtFactor=0;
} 


// ************************************************************************ 
// PUBLIC METHODS -- ANYONE MAY READ/WRITE 
// ************************************************************************ 
Person.prototype.beCool = function(){ this.clothing="khakis and black shirt" } 
Person.prototype.shower = function(){ this.dirtFactor=2 } 
Person.prototype.showLegs = function(){ alert(this+" has "+this.legs+" legs") } 
Person.prototype.amputate = function(){ this.legs-- } 


// ************************************************************************ 
// PROTOTYOPE PROERTIES -- ANYONE MAY READ/WRITE (but may be overridden) 
// ************************************************************************ 
Person.prototype.legs=2;


// ************************************************************************ 
// STATIC PROPERTIES -- ANYONE MAY READ/WRITE 
// ************************************************************************ 
Person.population = 0;



// Here is the code that uses the Person class 
function RunGavinsLife(){ 
    var gk=new Person("Gavin","caucasian");       //New instance of the Person object created. 
    var lk=new Person("Lisa","caucasian");        //New instance of the Person object created. 
    alert("There are now "+Person.population+" people");

    gk.showLegs(); lk.showLegs();                 //Both share the common 'Person.prototype.legs' variable when looking at 'this.legs' 

    gk.race = "hispanic";                         //Sets a public variable, but does not overwrite private 'race' variable. 
    alert(gk+"'s real race is "+gk.getRace());    //Returns 'caucasian' from private 'race' variable set at create time. 
    gk.eat(); gk.eat(); gk.eat();                 //weight is 3...then 9...then 27 
    alert(gk+" weighs "+gk.weigh()+" pounds and has a dirt factor of "+gk.dirtFactor);

    gk.exercise();                                //weight is now 13.5 
    gk.beCool();                                  //clothing has been update to current fashionable levels 
    gk.clothing="Pimp Outfit";                    //clothing is a public variable that can be updated to any funky value 
    gk.shower();
    alert("Existing shower technology has gotten "+gk+" to a dirt factor of "+gk.dirtFactor);

    gk.muchTimePasses();                          //50 Years Pass 
    Person.prototype.shower=function(){           //Shower technology improves for everyone 
        this.dirtFactor=0;
    } 
    gk.beCool=function(){                         //Gavin alone gets new fashion ideas 
        this.clothing="tinfoil";
    };

    gk.beCool(); gk.shower();
    alert("Fashionable "+gk+" at " 
        +gk.getAge()+" years old is now wearing " 
        +gk.clothing+" with dirt factor " 
        +gk.dirtFactor);

    gk.amputate();                                //Uses the prototype property and makes a public property 
    gk.showLegs(); lk.showLegs();                 //Lisa still has the prototype property 

    gk.muchTimePasses();                          //50 Years Pass...Gavin is now over 100 years old. 
    gk.eat();                                     //Complains about extreme age, death, and inability to eat. 
}

From:JavaScript Public And Private Properties And Methods - MySQL, Oracle Abc Wiki

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