匹配當前獲取焦點的元素。
如同其他偽類選擇器（那些以":"開始），建議:focus前面用標記名稱或其他選擇;否則，通用選擇("*")是不言而喻的。換句話說，$(':focus')等同為$('*:focus')。如果你正在尋找當前的焦點元素，$( document.activeElement )將檢索，而不必搜索整個DOM樹。
實例:
<!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>