js学习

一  java面向对象的5种写法

      ①

      function Circle(r){

      this.r = r;

  }

  Circle.PI = 14159;

  Circle.prototype.area = function(){

      return Circle.PI*this.r*this.r;

  }

     

  var c = new Circle(1.0);

  alert(c.area());               

    

     ②

     var Circle = function(){

      var obj = new Object();

      obj.PI = 3.14159;

           

      obj.area = function(r){

            return this.PI*r*r;    

      }

      return obj;

  }

     

  vary c = new Circle();

  alert(c.area(1.0));

 

     ③

     var Circle = new Object();

  Circle.PI = 3.14159;

  Circle.area = function(r){

      return this.PI*r*r;

  }

     

  alert(Circle.area(1.0));

    

     ④

     var Circle = {

      "PI":3.14159,

      "area":function(r){

            return this.PI*r*r;

      }

  };

 

  alert(Circle.area(1.0));

 

     ⑤

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

 

   alert(new Circle().area(1.0));

 

二 变量隐式定义

<script language="javascript">
(function test(){
   var a=b=5;
   alert(typeof(a))
   alert(typeof(b))
})()
   alert(typeof(a))
   alert(typeof(b))
</script>

输出：number number undefined number

var a=b=5
只定义a变量,
b变量未定义,所以隐式定义,在js中隐式定义是全局变量