Javascript概念

1. 判断属性是否存在或有值
   

   // BAD: This will cause an error in code when foo is undefined 
   if (foo) { 
   　　doSomething(); 
   } 
   // GOOD: This doesn't cause any errors. However, even when 
   // foo is set to NULL or false, the condition validates as true 
   if (typeof foo != "undefined") { 
   　　doSomething(); 
   } 
   // BETTER: This doesn't cause any errors and in addition 
   // values NULL or false won't validate as true 
   if (window.foo) { 
   　　doSomething(); 
   } 
   // UGLY: we have to proof existence of every 
   // object before we can be sure property actually exists 
   if (window.oFoo && oFoo.oBar && oFoo.oBar.baz) { 
   　　doSomething(); 
   }

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