Debugging Tips

When you're developing a gadget to run inside Google Wave, debugging can be hard. Why? Well, first of all, debugging on the web is hard generally. A bug could exist in your JavaScript, CSS, HTML, or server, and a bug might only manifest itself on one browser or operating system. On top of that, debugging gadgets is harder because everything is contained inside an iframe on a parent page, and you have to make sure the bug actually originates from your iframe and not the rest of the page. And then, finally, a Wave gadget is "Wave-y" because it involves syncing states across multiple participants, and now you have to figure out if the bug is in the state sharing, and if so, try to replicate that bug by simulating a sequence of participant interactions.

So, yes, debugging can be hard - but there are ways to make it easier. If you're new to debugging on the web, read this article on using Firebug for debugging. If you're new to debugging gadgets, read this article from the Gadget's developer guide. And if you're new to debugging Wave gadgets, well, you're in the right place.

This article will step through a variety of suggestions that will make gadget debugging easier. Note that the Google Wave sandbox as well as the Google Wave APIs are in preview mode, and we hope to create tools to make debugging easier in the future and update this article accordingly.

You can jump to individual sections using this table of contents:

Reloading New Code

Many gadget containers offer developer tools to reload a gadget for testing purposes, so that the developer does not have to reload the entire page. The Google Wave client does not offer any similar mechanism, so you have two options for reloading your gadget after you've changed the code:

  • If using Firefox, you can right-click on any part of the gadget and click "Reload iframe". This will take a few seconds to just reload your gadget, and not the rest of the page.
  • If you're not using Firefox, there are a few possibilities:
    • You can reload the entire page. To make the page load faster, maximize the window for the wave so that all of the other windows are closed.
    • You can load a different Wave than the one you are debugging (any arbitrary one will do), and then switch back to the original one. To make this technique faster, pick a second wave that's short. Or, you could even have the gadget in both waves, and just switch back and forth. Your gadget may hold different states in each instance, but that may be fine for what you are debugging.

Simulating Multiple Users

Since the power of Wave comes from collaboration amongst multiple participants, you are most likely creating a gadget that responds to interaction from participants. This collaborative model means that debugging your gadget can be hard, particularly if your gadget expects high-speed interaction. Ideally, you would always be sitting next to a willing tester, and you could watch them as you both simultaneously use the gadget on different computers.

Assuming this is not an option, an acceptable alternative is for you to open your gadget while logged into two different accounts. You can register for multiple accounts, like "" and "", and then use them both simultaneously. You may log into 2 different Wave accounts using several techniques:

  • Use two different computers. If you happen to have a desktop/laptop or other multi-computer setup, you might prefer this technique. 
  • Use two different browsers. For example, if you use Firefox as your main browser, then you could open the gadget in Chrome or Safari at the same time.
  • Use two profiles with the same browser. Firefox has a feature for power developers that lets them create multiple profiles and open up multiple instances of FF at the same time, provided each has a different profile. See Mozilla's article on Managing Profiles.
  • Use multiple cookies in the same browser instance. A third-party extension for Firefox called CookiePie exists that lets you use different cookies in each browser tab, meaning that you can log in to 2 different Wave accounts in the same browser. Download this extension from the Nekstra website.
  • Use Chrome in "incognito mode". In this mode, cookies are wiped clean for each new window, so it is possible to login to multiple accounts in two different tabs.

In every technique except the first, you should resize both browser windows to have 50% width so that you can quickly click on each, and see what's happening in both at the same time.

Generally, the advantage of using the same browser, like in the last two techniques, is that your debugging environment is the same and you can safely eliminate cross-browser issues as the cause of discrepancies between each instance of the gadget. On the other hand, after you have debugged the fundamental flow of your gadget, you may specifically want to test the gadget in multiple browsers at once, to check that you haven't introduced any cross-browser issues. The best debugging approach likely involves a mix of the above techniques at different stages in the development cycle, and every so often, begging a friend or family member to be your live tester.

Debug Messages

Programmers commonly output messages to the screen to indicate the program and variable state. JavaScript programmers commonly make the mistake of using the window.alert() function to output these messages. This function is used because it's simple to call and exists in all browsers, but it's simplicity is also dangerous. An alert() can change the flow of a JavaScript program, particularly one that involves any asynchronous callback functions like Wave gadgets, so the alert() can both make you think your program works when it doesn't, and make you think your program doesn't work when in fact it does. In addition, each alert() prompt must be formally closed by the user, making debugging quite slow, particularly if you accidentally code an infinite loop.

Thankfully, many of the modern browsers (Safari, Chrome, Firefox w/Firebug) now offer a console.log() function, and it will print everything from strings to objects without blocking program execution.

console.log("The score is now: " + score);

If you're using a browser that doesn't support that function, you can write string-based messages to a DIV on the screen. This may slightly slow down your program execution, but if you keep the number and length of messages to a minimum, then the impact shouldn't be significant.

document.getElementById("debugDiv").innerHTML += "The score is now: " + score;
<div id="debugDiv" style="width:100%; height:200px; overflow:scroll"></div>

The Gadgets API provides a MiniMessages library that makes writing strings to a DIV even easier, and adds a few features like close buttons and timer-based closing. To use this library, you must add another <Require> tag under the one that requests the wave-preview library, construct a new instance of the MiniMessage class with a designated DIV, and then call one of several create-message functions:

<Require feature="minimessage"/>
var msg = new gadgets.MiniMessage(__MODULE_ID__, document.getElementById("messageBox"));
msg.createDismissibleMessage("The score is now: " + score);

Since you may be testing your gadget on browsers that do not support the console functions, or you may be asking non-developers to test your gadget for you, you might consider a solution that mixes the two options. The function below uses console.log() if supported, and falls back to MiniMessages if not:

function logThis(obj) {
   if (window.console && console.log) {
   } else {

The screenshot below shows the output debug messages when using the MiniMessage library:

Conditional Debugging

When you are beginning your gadget development, you will likely want to be in a constant debug mode and always see your debug output. When your gadget is mostly done or ready for general use, you will want to turn off debug output for your users, but allow you to turn this output on when necessary for debugging. Web developers often achieve this conditional debugging by setting a debug flag on their URL, and using the existence of that flag to turn on debug mode. 

Gadget developers cannot set a debug flag through the URL, but they can get a similar result by creating and setting a user preference in the gadget XML. The example below creates a Boolean-type preference called "debugMode", which you can set to true when in the development stage, and to false once deployed to consumers.

<UserPref name="debugMode"
                      display_name="Debug Mode"

To use the value of that preference, you must construct a new instance of the Prefs() class, then retrieve the boolean value of the preference, and check for this value within your code when outputting debug messages.

prefs = new gadgets.Prefs();
var debugMode = prefs.getBool("debugMode");

You can then insert a line in your logging function that only logs if the debug mode boolean is true:

if (debugMode) {

Currently, the Google Wave client does not offer an interface for specifying user preferences when adding a gadget to a Wave through the Debug->Extensions menu, but it does offer a text box to specify the preferences as a JSON object. For instance, if you wanted to test your gadget with debugging off, then you could pass in {"debugMode": "true"}. Note that although the debugMode preference is a boolean, the value is passed in as a string.

Wave Debug Log

The Google Wave sandbox client comes with its own debug log, which the development team uses for troubleshooting issues, but can also be used by gadget developers for monitoring changes in the gadget state.

To enable the debug log, you first need to insert "?ll=debug" after the last slash in the URL and before the first "#":!w%252Bbwihmm04%2525B.1

Reload the page after changing the URL. From the Debug Menu, choose the newly listed option at the top called Show debug log. A console will pop up underneath the currently opened wave. To filter the output of that console to only show gadget-relevant messages, click gadgets. While that console is open, you will see all of the state changes that the gadget sends. You can clear or close the console at any time.

The screenshot below shows sample output from the "Are you in?" gadget:

In addition to seeing state changes, you can also output messages to this log using the wave.log() function, and each message will be prefixed with a timestamp:

wave.log("Storing state");

You can add this utility method to the arsenal of message outputting techniques from the above section. Note that this function can only output strings, whereas the console.log() function can output any JavaScript object in a way that makes them easier to inspect, so wave.log() is more equivalent in utility to the write-to-DIV techniques.

Storing All States

The above techniques can be useful when debugging the state changes in one view of a gadget, but it can be tricky to debug the state changes happening simultaneously in two views of a gadget, particularly if they happen very quickly.

One way of debugging the sequence and content of state changes from all participants in a gadget is to save those state changes to a file or database on a server.

Before doing any state saving, you first need to assign an identifier to each gadget "session" - i.e. there should be a different number for each Wave that a gadget is embedded in. This identifier will allow you to save per-Wave logs, reflecting only the sequences of state for the gadget in one Wave, but for all participants on that gadget. You should generate the identifier when the gadget is first embedded in the Wave &mdash when the first participant is viewing the gadget for the first time. You can use the Date() class to generate a unique ID, and then save that ID into the state, so that all participants will see it.

var newDate = new Date();
var stateDelta = {"sessionId": ("" + newDate.getTime() + "")};

Now, everytime that you submit a state delta, you should save that delta into a file on your server, where the filename includes the unique identifier. To send the delta to your server, you can use the Gadgets API makeRequest() function, and pass the ID and a stringifed version of the state delta as POST parametrs.

function submitDelta(state, stateDelta) {
  var url = "";
  var params = {};
  postdata ={
          id: state.get('sessionId') || stateDelta['sessionId'], 
          state: JSON.stringify(stateDelta)});
  params[] =;
  params[]= postdata;, function(){}, params); 

To save the delta to your server, you can setup a script that retrieves the state and ID from the POST parameters, and appends the state to the file identified by the ID. If the file doesn't exist yet (if it is the first submission of a state delta), then the script needs to create the file first.

$state = $_POST["state"];
$id = $_POST["id"];
$filename = "wavestate_" . $id . ".txt";
$fp = fopen($filename, "a+");
fwrite($fp, "\"" . $state . "\",\n");

Now, whenever you are testing your gadget, you can load the debug file for the current ID and see what deltas are being submitted by various participants. This technique is particularly helpful if you're testing with participants that are located remotely.

Note that you could attain similar debug information using a database and a language like Python or Java; the PHP file-writing code is just one possible variation.

Re-playing State Changes

When you're developing a gadget and debugging, you may find yourself tire of constant gadget interaction to trigger state changes. Conveniently, if you followed the design pattern above of storing state to your server, you can re-play those state changes in your gadget, and save yourself the effort.

First, create a few form fields and a button in your gadget, so that you can enter the desired session ID and speed for playback. Connect a function to the button that will load the state deltas to playback:

<div id="testBox" style="display:none">
    Id: <input id="idText" type="text" value="1251874497293"/>
    MS: <input id="speedText" type="text" value="2000"/>
    <input id="testButton" type="button" value="Test" onclick="loadStateDeltas()" disabled/>

Then, inside that function, load the state deltas for the given ID by requesting them in JSONP form from your server. This function first needs to restate the state in your gadget, and that can be done by submitting a state delta where each key is set to null.

function loadStateDeltas() {
  var id = document.getElementById("idText").value;
  var script = document.createElement("script");
  script.src = "" + id;
function resetState() {
  var state = wave.getState();
  wave.setStateCallback(function() {});
  var stateDelta = {"key1": null, "key2": null, "key3": null};

The PHP script on your server can read the file and pass the concatenated deltas as a JavaScript array into a callback function which you'll define in your Gadget code:

$id = $_GET["id"];
$deltas = file_get_contents($filename);
echo "onStateDeltasLoaded([" . $deltas . "]);";

The callback function submits each state delta using window.setInterval at the given speed:

function onStateDeltasLoaded(stateDeltas) {
  var speed = parseInt(document.getElementById("speedText").value);
  var state = wave.getState();
  var currentNum = 0;
  window.setInterval(function() {
   }, speed);

Once you've implemented the ability to re-play state, you might want to enable the re-play functionality using user preferences as noted above in Conditional Debugging.


You may find yourself using only some of these suggestions, using all of them, or coming up with your own variations. The techniques you use depend largely on the type of gadget that you're creating, and your typical development environment. Either way, please do share your ideas and feedback in the Google Wave APIs discussion forum.