JavaScript: How to detect the browser currently used by the user?

Opera

var isOpera = (!!window.opr && !!opr.addons) || !!window.opera || navigator.userAgent.indexOf(' OPR/') >= 0;

Firefox 1.0+

var isFirefox = typeof InstallTrigger !== 'undefined';

At least Safari 3+: “[object HTMLElementConstructor]”

var isSafari = Object.prototype.toString.call(window.HTMLElement).indexOf('Constructor') > 0;

Internet Explorer 6-11

var isIE = /*@cc_on!@*/false || !!document.documentMode;

Edge 20+

var isEdge = !isIE && !!window.StyleMedia;

Chrome 1+

var isChrome = !!window.chrome && !!window.chrome.webstore;