实例代码“Ctrl+/”提示“F11/ESC”全屏 返回 格式化 恢复 运行
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<!DOCTYPE html>
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<html>
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    <head>
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        <title>
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            Undefined Example 3
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        </title>
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        <script type="text/javascript">
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            var message; //this variable is declared but has a value of undefined
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            //make sure this variable isn't declared
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            //var age
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            alert(message); //"undefined"
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            alert(age); //causes an error
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        </script>
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    </head>
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    <body>
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    </body>
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</html>