JavaScript中var声明导致undefined的陷阱与变量作用域解析

示例代码结构

<!DOCTYPE html> 
<html> 
  <head> 
    <meta charset="utf-8"> 
    <title>js中全局变量与局部变量</title> 
  </head> 
  <body> 
    <script type="text/javascript"> 
      var a = "全局变量"; 
      function test1() 
      { 
        var a = "局部变量"; 
         
        alert(a); 
      } 
       
      function test2() 
      { 
        alert(a); 
         
        var a = "局部变量"; 
         
        alert(a); 
      } 
       
      function test3() 
      { 
        alert(a); 
         
        a = "局部变量"; 
         
        alert(a); 
      } 
    </script> 
     
    <input type="button" value="test1" onclick="test1()"/> 
    <input type="button" value="test2" onclick="test2()"/> 
    <input type="button" value="test3" onclick="test3()"/> 
 
  </body> 
</html>