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Integrating a JavaScript library as an extension

JavaScript can also be used for creating Extensions e.g. for integrating existing JavaScript libraries. See Creating a UI extension for general information about Extensions. The main difference when using JavaScript is that you extend AbstractJavaScriptExtension, that your shared state class should extend JavaScriptExtensionState and then of course that your client-side implementation is written in JavaScript. See Integrating a JavaScript component for basic information about how to use JavaScript for your client-side logic.

This tutorial will create a simple Extension for integrating Google Analytics. Because the Analytics API just uses the same _gaq.push function with different arguments, the JavaScript connector logic can be equally simple. Aside from asynchronously loading ga.js, the client-side code just adds a callback that the server-side code can use to push new commands.

window._gaq = window._gaq || [];

(function() {
  var ga = document.createElement('script');
  ga.type = 'text/javascript';
  ga.async = true;
  ga.src = ('https:' == document.location.protocol ?
    'https://ssl' : 'http://www') +
  var s = document.getElementsByTagName('script')[0];
  s.parentNode.insertBefore(ga, s);

window.com_example_Analytics = function() {
  this.pushCommand = function(command) {

The server-side Extension class provides the common Extension API for extending a UI instance as well as API for some Analytics features. All the Analytics features are based on the pushCommand method that invokes the corresponding client-side callback.

The Analytics API used in this example has nothing that warrants using shared state, but you can of course use shared state in your own JavaScript Extension if you want to as long as your state class extends JavaScriptExtensionState.

public class Analytics extends AbstractJavaScriptExtension {
  public Analytics(UI ui, String account) {
    pushCommand("_setAccount", account);

  public void trackPageview(String name) {
    pushCommand("_trackPageview", name);

  private void pushCommand(Object... commandAndArguments) {
    // Cast to Object to use Object[] commandAndArguments as the first
    // varargs argument instead of as the full varargs argument array.
    callFunction("pushCommand", (Object) commandAndArguments);

Extensions are suitable for integrating many existing JavaScript libraries that do not provide a component that is added to a layout. By using a client-side JavaScript connector for integrating the JavaScript library, you can eliminate GWT from the equation to give you slightly less code to maintain.