浏览器报错:未定义和undifined不是同一概念，前者是没有申明，后者是没有赋值。
1:
<html>
<body> <script> if(!("a" in window)){ var a = 1; } alert(a);//undifined function b(){ var c = 2; } alert(b);//函数b //alert(c);//报错 c未定义 alert("c" in window);//false alert(c);//报错 c未定义 </script> </body> </html> 2:<html>
<body> <script> var a = 1 b = function a(x){ x && a(--x); }; alert(a);//1 </script> </body> </html>3:
<html>
<body> <script> if(!("" in window)){ var a = 1; } alert(a);//1 </script> </body> </html>4:
<html>
<body> <script> var d; alert(d);//undifined var a = 1; b = function a(x){ x && a(--x); }; alert(a);//1 </script> </body> </html>
@江西-小若 总结