匹配当前获取焦点的元素。
如同其他伪类选择器（那些以":"开始），建议:focus前面用标记名称或其他选择;否则，通用选择("*")是不言而喻的。换句话说，$(':focus')等同为$('*:focus')。如果你正在寻找当前的焦点元素，$( document.activeElement )将检索，而不必搜索整个DOM树。
实例:
<!DOCTYPE html> <html lang='zh-cn'> <head> <title>Insert you title</title> <meta http-equiv='description' content='this is my page'> <meta http-equiv='keywords' content='keyword1,keyword2,keyword3'> <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> <script type='text/javascript' src='./js/jquery-3.0.0.js'></script> <style type='text/css'> input{padding-left:8px;font:400 13px/1.2em 'Courier New';} .focused{background:#E9B;color:#000;outline:none;} </style> <script type='text/javascript'> $(function(){ $("input:focus").addClass('focused'); }); </script> </head> <body> <form id='formDemo'> <input type='text' value='user' autofocus='on'/> <input type='password' value='password' autofocus='off'/> </form> </body> </html>