The Navigator.userAgent read-only property returns the user agent string for the current browser.

Browser identification based on detecting the user agent string is unreliable and is not recommended, as the user agent string is user configurable. For example:

• In Firefox, you can change the preference general.useragent.override in about:config. Some Firefox extensions do that; however, this only changes the HTTP header that gets sent and that is returned by navigator.userAgent. There might be other methods that utilize JavaScript code to identify the browser.
• Opera 6+ allows users to set the browser identification string via a menu.

A string specifying the complete user agent string the browser provides both in HTTP headers and in response to this and other related methods on the Navigator object.

The user agent string is built on a formal structure which can be decomposed into several pieces of info. Each of these pieces of info comes from other navigator properties which are also settable by the user. Gecko-based browsers comply with the following general structure:

userAgent = appCodeName/appVersion number (Platform; Security; OS-or-CPU;
Localization; rv: revision-version-number) product/productSub
Application-Name Application-Name-version

• User-Agent HTTP header