Returns the user agent string for the current browser.


var ua = window.navigator.userAgent;


ua stores the user agent string value of the current browser.

The window.navigator.userAgent property is read-write; it has no default value.

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


// alerts "Mozilla/5.0 (Windows; U; Win98; en-US; rv:0.9.2) Gecko/20010725 Netscape6/6.1"


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 doesn't affect browser detection performed by JavaScript code.
  • Opera 6+ allows users to set the browser identification string via a menu
  • Microsoft Internet Explorer uses the Windows registry
  • Safari and iCab allow users to change the browser user agent string to predefined Internet Explorer or Netscape strings via a menu.


DOM Level 0. Not part of any standard.

See also

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 Last updated by: teoli,