I think you're looking for navigator.doNotTrack
:
console.log(window.navigator.doNotTrack);
// prints "yes" if DNT is enabled; otherwise this is "unspecified" in Firefox
MDN explains that in Firefox:
When the do-not-track header sends "1", navigator.doNotTrack
is "yes". When the header is unset, navigator.doNotTrack
is "unspecified". When the header sends "0" (currently unsupported in Firefox), navigator.doNotTrack
is "no".
In other browsers:
IE9, Opera 12, and Safari 5.1 are based on an earlier version of this specification where navigator.doNotTrack is the value sent for the do-not-track header.
IE9 uses a vendor prefix, i.e., navigator.msDoNotTrack
So, you might detect DNT in general by doing:
var isDNT = navigator.doNotTrack == "yes" || navigator.doNotTrack == "1" ||
navigator.msDoNotTrack == "1";