为什么在外部范围内定义时,阴影变量评估为未定义? [英] Why does shadowed variable evaluate to undefined when defined in outside scope?

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问题描述

考虑以下代码:

<html><head></head>
<body>
    <script type="text/javascript">
        var outside_scope = "outside scope";
        function f1() {
            alert(outside_scope) ;
        }
        f1();
    </script>
</body>
</html> 

此代码的输出是警告框显示消息outside范围".但是,如果我将代码稍微修改为:

The output for this code is that the alert box displays the message "outside scope". But, if I slightly modify the code as:

<html><head></head>
<body>
    <script type="text/javascript">
        var outside_scope = "outside scope";
        function f1() {
            alert(outside_scope) ;
            var outside_scope = "inside scope";
        }
        f1();
    </script>
</body>
</html> 

警报框显示消息未定义".我本可以有如果在两种情况下都显示未定义",则理解逻辑.但是,那个没有发生.它仅在第二种情况下显示未定义".这是为什么?

the alert box displays the message "undefined". I could have understood the logic if it displays "undefined" in both the cases. But, that is not happening. It displays "undefined" only in the second case. Why is this?

预先感谢您的帮助!

推荐答案

在第一种情况下,您的代码正在访问全局变量outside_scope",该变量已被初始化为outside scope".

In the first case, your code is accessing the global variable "outside_scope", which has been initialized to "outside scope".

Javascript 具有函数级作用域,因此在第二种情况下,它访问的是函数作用域变量outside_scope",但在出现警告框时尚未初始化.所以它显示未定义.

Javascript has function level scope, so in the second case it is accessing the function scoped variable "outside_scope", but it has not yet been initialized at the time of the alert box. So it displays undefined.

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