+JS is a Javascript prefix class. When a +Button uses this prefix, it will POST the page using Javascript. This may lead to a faster response and avoids flicker caused by reloading and rendering the page. More information is available in the application tutorial. See also +Button.
This field can be used to hold Javascript-driven context. Setting Google Maps images would be one example:

    (gui '(+JS +Button) "@img/mapicon.gif" "@img/no.png"
       '(set> (: home map) (path> (: home obj) 40)) )
    (<p> '((id . map) (style . "width: 600px; height: 400px"))
       (gui 'map '(+JsField) "showMap") )
See also +gui.