Enforcing new 模式

  
  
  
  

   
   
   
    // 构造函数
  
  
  
  
  
  
  
  
			
   
   
   
   function 
   
   
   
   Waffle
   
   
   
   () 
   
   
   
   {
  
  
  
  
  
  
  
  
				
   
   
   
   this
   
   
   
   .
   
   
   
   tastes 
   
   
   
   = 
   
   
   
   "yummy"
   
   
   
   ;
  
  
  
  
  
  
  
  
			
   
   
   
   }
  
  
  
  
  
  
  
  


  
  
  
  
  
  
  
  
			
   
   
   
   // 反例
  
  
  
  
  
  
  
  
			
   
   
   
   // 忘记使用 `new`
  
  
  
  
  
  
  
  
			
   
   
   
   var 
   
   
   
   good_morning 
   
   
   
   = 
   
   
   
   Waffle
   
   
   
   ();
  
  
  
  
  
  
  
  
			
   
   
   
   console
   
   
   
   .
   
   
   
   log
   
   
   
   (
   
   
   
   typeof 
   
   
   
   good_morning
   
   
   
   ); 
   
   
   
   // "undefined"
  
  
  
  
  
  
  
  

   
   
   
    
   
   
   
   console
   
   
   
   .
   
   
   
   log
   
   
   
   (
   
   
   
   window
   
   
   
   .
   
   
   
   tastes
   
   
   
   );
   
   
   
    
   
   
   
   // "yummy" 此时
   
   
   
   构造函数Waffle中的this指代window
  
  
  
  
  
  
  
  


  
  
  
  
  
  
  
  


  
  
  
  
  
  
  
  
			
   
   
   
   //更改为使用new关键字
  
  
  
  
  
  
  
  
			
   
   
   
   var 
   
   
   
   good_morning 
   
   
   
   = 
   
   
   
   new 
   
   
   
   Waffle
   
   
   
   ();
  
  
  
  
  
  
  
  
			
   
   
   
   console
   
   
   
   .
   
   
   
   log
   
   
   
   (
   
   
   
   typeof 
   
   
   
   good_morning
   
   
   
   ); 
   
   
   
   // "object"
  
  
  
  
  
  
  
  
			
   
   
   
   console
   
   
   
   .
   
   
   
   log
   
   
   
   (
   
   
   
   good_morning
   
   
   
   .
   
   
   
   tastes
   
   
   
   ); 
   
   
   
   // "yummy"
  
  
  
  
  
  
  
  


  
  
  
  
  
  
  
  


  
  
  
  
  
  
  
  
			
   
   
   
   // 强制使用new
  
  
  
  
  
  
  
  
			
   
   
   
   function 
   
   
   
   Waffle
   
   
   
   () 
   
   
   
   {
  
  
  
  
  
  
  
  
				
   
   
   
   if 
   
   
   
   (
   
   
   
   !
   
   
   
   (
   
   
   
   this 
   
   
   
   instanceof 
   
   
   
   Waffle
   
   
   
   )) 
   
   
   
   {
  
  
  
  
  
  
  
  
					
   
   
   
   return 
   
   
   
   new 
   
   
   
   Waffle
   
   
   
   ();
  
  
  
  
  
  
  
  
				
   
   
   
   }
  
  
  
  
  
  
  
  


  
  
  
  
  
  
  
  
				
   
   
   
   this
   
   
   
   .
   
   
   
   tastes 
   
   
   
   = 
   
   
   
   "yummy"
   
   
   
   ;
  
  
  
  
  
  
  
  
			
   
   
   
   }
  
  
  
  
  
  
  
  


  
  
  
  
  
  
  
  
			
   
   
   
   var 
   
   
   
   good_morning 
   
   
   
   = 
   
   
   
   new 
   
   
   
   Waffle
   
   
   
   ();
  
  
  
  
  
  
  
  
			
   
   
   
   var 
   
   
   
   good_evening 
   
   
   
   = 
   
   
   
   Waffle
   
   
   
   ();
  
  
  
  
  
  
  
  


  
  
  
  
  
  
  
  
			
   
   
   
   console
   
   
   
   .
   
   
   
   log
   
   
   
   (
   
   
   
   typeof 
   
   
   
   good_morning
   
   
   
   ); 
   
   
   
   // "object"
  
  
  
  
  
  
  
  
			
   
   
   
   console
   
   
   
   .
   
   
   
   log
   
   
   
   (
   
   
   
   good_morning
   
   
   
   .
   
   
   
   tastes
   
   
   
   ); 
   
   
   
   // "yummy"
  
  
  
  
  
  
  
  
			
   
   
   
   console
   
   
   
   .
   
   
   
   log
   
   
   
   (
   
   
   
   typeof 
   
   
   
   good_evening
   
   
   
   ); 
   
   
   
   // "object"
  
  
  
  
  
  
  
  
			
   
   
   
   console
   
   
   
   .
   
   
   
   log
   
   
   
   (
   
   
   
   good_evening
   
   
   
   .
   
   
   
   tastes
   
   
   
   ); 
   
   
   
   // "yummy"