js multiple browsers judge code separately_javascript skills

/* 浏览器判断 */
var userAgent = navigator.userAgent.toLowerCase();
var is_webtv = userAgent.indexOf('webtv') != -1;
var is_kon = userAgent.indexOf('konqueror') != -1;
var is_mac = userAgent.indexOf('mac') != -1;
var is_saf = userAgent.indexOf('applewebkit') != -1 || navigator.vendor == 'Apple Computer, Inc.';
var is_opera = userAgent.indexOf('opera') != -1 && opera.version();
var is_moz = (navigator.product == 'Gecko' && !is_saf) && userAgent.substr(userAgent.indexOf('firefox') 8, 3);
var is_ns = userAgent.indexOf('compatible') == -1 && userAgent.indexOf('mozilla') != -1 && !is_opera && !is_webtv && !is_saf;
var is_ie = (userAgent.indexOf('msie') != -1 && !is_opera && !is_saf && !is_webtv) && userAgent.substr(userAgent.indexOf('msie') 5, 3);