问题：捕获document对象的keydown事件，再通过调用document.writeln函数输出当前按键

思路1：捕获keydown事件，然后从该事件的which属性中获得当前按键Unicode格式的键码

1. 
   
2. 
   
3. window.onload=function{
   
4.     if ( document.addEventListener ){
   
5.           document.addEventListener( "keydown", getKey , false );
   
6.     }else if ( document.attachEvent ){
   
7.           document.attachEvent( "keydown" getKey );
   
8.      }
   
9. }
   
10. 
    
11. function getKey( event ){
    
12.      var theEvent = event ? event : window.event;
    
13.      document.writeln ( theEvent.which );
    
14. }
    
15. 
    

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学习了，虽然还不太懂