JavaScript 警报方法

Question

为什么以下行没有警报？

  <script>
  alert(x);//this line is not executed or alerted  ???.It should have alerted "undefined" 
  console.log(x)//reference error 
  var y="maizere";
  alert(y);//this line is also not executed or alerted  ???

  </script>

任何未声明的变量在 JavaScript 中都会被视为全局变量，对吧？

Answer 1

alert(x);//this line is not executed or alerted  ???.It should have alerted "undefined" 

这是不正确的。此行引发 ReferenceError，因为没有变量 x。

将此与情况进行对比:

var obj = {};
alert(obj.x); // undefined - there's no attribute x