jQuery judges the browser

<script language="JavaScript" type="text/javascript">
    if ((navigator.userAgent.indexOf('MSIE') >= 0)
            && (navigator.userAgent.indexOf('Opera') < 0)){
        alert('你是使用IE')
    }else if (navigator.userAgent.indexOf('Firefox') >= 0){
        alert('你是使用Firefox')
    }else if (navigator.userAgent. indexOf ( 'Opera' ) >= 0 ){
         alert ( 'You are using Opera' ) 
    } else {
         alert ( 'You are using another browser to browse the web!' ) 
    } </ script >