[NTG-context] JavaScript controlled button appearance

Dear list, I have the following sample: \setupinteraction[state=start] \setupbodyfont[heros] \startJSpreamble varia used now function SwitchFS() { app.fs.isFullScreen = !app.fs.isFullScreen; } \stopJSpreamble \starttext \startTEXpage[offset=10pt] \goto{fs}[JS(SwitchFS{})] \stopTEXpage \stoptext How can I get that the "fs" in the text is replaced with "x" when JavaScript detects that the screen is in full screen mode? Many thanks for your help, Pablo -- http://www.ousia.tk