js获取页面选中文字并显示为什么不对，哪里错啦。

<!DOCTYPE html> <html> <head> <title>2.html</title> <meta http-equiv="keywords" content="keyword1,keyword2,keyword3"> <meta http-equiv="description" content="this is my page"> <meta http-equiv="content-type" content="text/html; charset=UTF-8">  </head> <script type="text/javascript"> window.onload=function(){ document.getElementById('txt').onmouseup=function(){ var text=window.getSelection(); if(text.toString().length>1){ alert(text);} } } </script> <body id="txt"> This is my HTML page. <br> </body> </html> --mouseup首字母大写了

js获取页面选中文字并显示为什么不对，哪里错啦。

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