javascript面向对象的5种写法

定义Circle类，拥有成员变量半径r，常量PI和计算面积的成员函数area()

//第1种写法

function Circle(r) {  
   this.r = r;  
}  

Circle.PI = 3.14159;  

Circle.prototype.area = function() {
   return Circle.PI * this.r * this.r;  
}

var c = new Circle(1.0);  

alert(c.area());   

//第2种写法

var Circle = function() {   
  var obj = new Object();   
  obj.PI = 3.14159;   
  obj.area = function(r) {   
     return this.PI * r * r;   
  }
  return obj;   
}
  
var c = new Circle();   

alert(c.area(1.0));   

//第3种写法

var Circle = new Object();   
Circle.PI = 3.14159;   
Circle.Area = function(r) {   
   return this.PI * r * r;
}   

alert(Circle.Area(1.0));   

//第4种写法

var Circle={
  "PI":3.14159,   
  "area":function(r){   
           return this.PI * r * r;   
         }   
};   

alert(Circle.area(1.0));

//第5种写法   

var Circle = new Function("this.PI = 3.14159;this.area = function(r) {return r*r*this.PI;}");

alert((new Circle()).area(1.0));