Ok, so I was ruminating over this one. I just modified the default web project to the code you supplied and it worked fine. Not sure what is different between my code and yours.
The JSO class package com.testbrowser.client; import com.google.gwt.core.client.JavaScriptObject; public class BrowserVersionList extends JavaScriptObject { protected BrowserVersionList() { } public static BrowserVersionList newInstance() { return createJso(); } private static native BrowserVersionList createJso() /*-{ return $wnd.brVersionMap; }-*/; public final native String getChromeVersion() /*-{ return this.Chrome; }-*/; } The version js file: brVersionMap={"IE":"8","Firefox":"15","Chrome":"17","Safari":"5"}; The main onModuleLoad body to show the version found: public class BrowserSupportVersion implements EntryPoint { /** * This is the entry point method. */ public void onModuleLoad() { BrowserVersionList browserVersions = BrowserVersionList.newInstance(); GWT.log("Chrome version supported: "+browserVersions.getChromeVersion()); // Create the popup dialog box final DialogBox dialogBox = new DialogBox(); dialogBox.setText("Detecting supported browser versions"); dialogBox.setAnimationEnabled(true); // We can set the id of a widget by accessing its Element final Label textToServerLabel = new Label(); VerticalPanel dialogVPanel = new VerticalPanel(); dialogVPanel.addStyleName("dialogVPanel"); dialogVPanel.add(new HTML("<b>Supported Chrome version: <i>"+browserVersions.getChromeVersion()+"</i></b>")); dialogVPanel.add(textToServerLabel); dialogVPanel.setHorizontalAlignment(VerticalPanel.ALIGN_RIGHT); dialogBox.setWidget(dialogVPanel); RootPanel.get("notificationContainer").add(dialogBox); } } I put this up and a version of my AppSpot account and it works from IE8, FF12, and Chrome 18. *http://4.runpartner.appspot.com/* Let me know if it works for you. Not sure what's causing your error. ( I cleaned the code up a little, but it ran as you had it too.) Sincerely, Joseph -- You received this message because you are subscribed to the Google Groups "Google Web Toolkit" group. To view this discussion on the web visit https://groups.google.com/d/msg/google-web-toolkit/-/kg1d0Lv3maoJ. To post to this group, send email to google-web-toolkit@googlegroups.com. To unsubscribe from this group, send email to google-web-toolkit+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/google-web-toolkit?hl=en.