Navigator: userAgent property

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

Note: The specification asks browsers to provide as little information via this field as possible. Never assume that the value of this property will stay the same in future versions of the same browser. Try not to use it at all, or only for current and past versions of a browser. New browsers may start using the same UA, or part of it, as an older browser: you really have no guarantee that the browser agent is indeed the one advertised by this property.

Also keep in mind that users of a browser can change the value of this field if they want (UA spoofing).

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. For more information about the form of the user agent string, see the User-agent HTTP header.


// alerts "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:124.0) Gecko/20100101 Firefox/124.0"


HTML Standard
# dom-navigator-useragent-dev

Browser compatibility

BCD tables only load in the browser

See also