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Monday, July 25, 2016

Automatic JS error detection with STF and WebDriver

If you followed my post on Catching java script errors with WebDriver then you already know how browser console logs can be used to catch javascript errors. But a problem which surfaces is that javascript error check has to be added on each test step where java script error check is to be done. This would result in lots of boilerplate code. A better approach would be to automate this check.

This is what STF 4.0.14 does. It introduces CustomEventListener class which catches SEVERE errors on browser console. But before delving into CustomEventListener, let’s understand two classes available from webdriver API -

EventFiringWebDriver implements WebDriver interface besides other interfaces. It supports registering WebDriverEventListener which can be used for logging purpose. For example get() method to open application URL looks as following in EventFiringWebDriver -

public void get(String url) {
dispatcher.beforeNavigateTo(url, driver);
dispatcher.afterNavigateTo(url, driver);

Hence it is a call to get method from WebDriver interface along with dispatcher statement before and after it. Herein beforeNavigateTo and afterNavigateTo are the methods in WebDriverEventListener interface. Other methods in EventFiringWebDriver follow the same structure, for ex click method looks as -

public void click() {
dispatcher.beforeClickOn(element, driver);;
dispatcher.afterClickOn(element, driver);

Herein WebDriverEventListener is the interface (described later) which does the job of logging or carrying out an operation you want before get(), click() or any other WebDriver operation.
There are two more important methods in EventFiringWebDriver which are used to register or unregister a WebDriverEventListener  -

public EventFiringWebDriver register(WebDriverEventListener eventListener) {
return this;

public EventFiringWebDriver unregister(WebDriverEventListener eventListener) {
return this;

This is a listener interface which should be implemented to catch WebDriver events. If you don’t want to implements all of the methods in this interface than you can extend AbstractWebDriverEventListener class and override those methods (or events) for which you want to collect logs

And this is what STF does. CustomEventListener extends AbstractWebDriverEventListener class and overrides navigation and click methods -

public void beforeNavigateTo(String url, WebDriver webDriver) {
  logErrors(url, getBrowserLogs(webDriver));

public void afterNavigateTo(String url, WebDriver webDriver) {
  logErrors(url, getBrowserLogs(webDriver));

public void beforeClickOn(WebElement element, WebDriver driver) {
  logErrors("before", element, getBrowserLogs(driver));

public void afterClickOn(WebElement element, WebDriver driver) {
  logErrors("after", element, getBrowserLogs(driver));
getBrowserLogs gets the log entries for browser -

private LogEntries getBrowserLogs(WebDriver webDriver) {
  return webDriver.manage().logs().get(LogType.BROWSER);

and logErrors() logs the Severe browser console errors -

private void logErrors(String url, LogEntries logEntries) {
  if (logEntries.getAll().size() == 0) {
      TestLogging.log("********* No Severe Error on Browser Console *********", true);
  } else {
      for (LogEntry logEntry : logEntries) {
          if (logEntry.getLevel().equals(Level.SEVERE)) {
              TestLogging.log("URL: "+url);
              TestLogging.logWebStep("Time stamp: " + logEntry.getTimestamp() + ", " +
                      "Log level: " + logEntry
                      .getLevel() + ", Log message: " + logEntry.getMessage(), true);
              isJSErrorFound = true;
      assert !isJSErrorFound;

And STF takes care of instantiating EventFiringWebDriver. But if you are not using STF then you can instantiate EventFiringWebDriver as following and use it in rest of your tests -

WebDriver driver =
CustomEventListener eventListener = new CustomEventListener();
EventFiringWebDriver eventFiringWebDriver = new EventFiringWebDriver(driver);

How do I use STF java script error check?

You should be using STF 4.0.14. Add property eventFiringWebDriver to your testng.xml file and set it to true -
<parameter name="eventFiringWebDriver" value="true"/>

That’s all and you are ready to catch all javascript errors from your tests. Simple is not it? :-)
I added a test in testng.xml file which demonstrate catching javascript errors with WebDriver. When you run test and open test report then you would find javascript error reported as following -

Screenshot from 2016-07-13 16:50:13.png

The test above uses the page with javascript error created by Alister Scott


  • Browser Logging mechanism is beta hence you may encounter unexpected behaviour.

  • I did not have great success in catching JS error on firefox browser. If you run the test from testng.xml file on firefox browser then no error is reported.

So what are you waiting for? Don’t let the JS error slip into production any more. Try it out and share your experience :)
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