Question

为什么不警告以下行？

  <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中被视为全局变量吗？

Answer 1

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

那是不对的。此行引发ReferenceError，因为没有变量x。

将此情况与情况进行对比：

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

Answer 2

您的undefined和undeclared混淆了。

变量x未声明 - 尚未在代码中声明，所以

alert(x);

会引发'x' is not declared之类的错误。

您的代码应该是

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

Answer 3

应该定义您在alert电话中放置的内容。如果不是（您的情况）则会抛出错误。因此，您应该首先定义变量。对于错误，您应该检查JavaScript（浏览器）控制台。

var x = "Test";
alert(x); // shows
var y = 123;
alert(y); // y also shows, because there is no error thrown inside the alert for x

javascript警报方法

3 个答案: