GWT JSNI Variables – An Exhaustive List

As anyone using the Google Web Toolkit JSNI knows, there are certain reserved variables, or “dollar words,” exposed by the GWT runtime. While you may be familiar with $wnd and $doc, there are more undocumented words. See the exhaustive (as of GWT 2.5.0) list below taken from source.


Reference to the root browser window GWT widgets are being rendered to. Note that when called from the iframe linker, GWT code runs in an iframe, not the root browser window, so calling window will reference the wrong window.


Same as above, but referencing the document, not window. Same as $wnd.document.


The X.Y.Z version of GWT this code was compiled with. Because Google uses the GWT trunk in their apps, they will return 0.0.999.


The name of the compiled GWT module. Note, you may have assumed this was obfuscated. It’s not.


The root URL for this module’s entrypoint.


The MD5 hash for this module.


Url and port of code server in hosted mode. i.e.

$hostedHtml Version

Version of hosted HTML page (hosted.html) in use. This is the page that shows error traces in browser in hosted mode.


Used when collecting stats in hosted mode. Same value as window.__gwt_SessionID


Undocumented. Used to collect stats in hosted mode.


Method that makes code reentry safe. This should wrap GWT code that will be called from outside of GWT via Javascript calls you wish to expose from your GWT application. See docs for more.

$stack, $stackDepth, $location

Undocumented. Trace information used by JsStackEmulator.

Example of accessing GWT Version

To determine the running version of a GWT application, you can use JSNI to return that version.

// easily get the GWT version
public static native String getGwtVersion() /*-{
    return $gwt_version;

From outside the application you can walk the DOM of the page to find the iframe containing the running GWT code and access the version like below.

var gwtVersion = null;
var frames = document.getElementsByTagName('iframe');
for (var i=0; i<frames.length; i++) {
    // prevent security access errors
    try {
        if(frames[i].contentWindow.$gwt_version) {
            gwtVersion = frames[i].contentWindow.$gwt_version;
    catch(e) {}

This is the mechanism I used to detect GWT apps in the Library Detector Chrome Extension (Github). Note the wrapping in a try/catch. This is because some iframes are cross domain and will throw a security exception if you try to access them.