JavaScript browser verification code (from discuz)_Basic knowledge

When looking at the source code of Discuz!, I found a piece of browser verification code, which is quite classic:

Copy code Code As follows:

var BROWSER = {};
var USERAGENT = navigator.userAgent.toLowerCase();
alert(USERAGENT);
BROWSER.ie = window.ActiveXObject && USERAGENT.indexOf('msie') != -1 && USERAGENT.substr(USERAGENT.indexOf('msie') 5, 3);
BROWSER.firefox = document.getBoxObjectFor && USERAGENT.indexOf('firefox') ! = -1 && USERAGENT.substr(USERAGENT.indexOf('firefox') 8, 3);
BROWSER.chrome = window.MessageEvent && !document.getBoxObjectFor && USERAGENT.indexOf('chrome') != -1 && USERAGENT.substr(USERAGENT.indexOf('chrome') 7, 10);
BROWSER.opera = window.opera && opera.version();
BROWSER.safari = window.openDatabase && USERAGENT.indexOf(' safari') != -1 && USERAGENT.substr(USERAGENT.indexOf('safari') 7, 8);
BROWSER.other = !BROWSER.ie && !BROWSER.firefox && !BROWSER.chrome && !BROWSER. opera && !BROWSER.safari;
BROWSER.firefox = BROWSER.chrome ? 1 : BROWSER.firefox;