Using WebDriver to automatically check for JavaScript errors on every page

Update: please see my newer post on this that doesn’t require changes to your application.

One of the benefits of using a page-object model is that you can perform certain actions on every page in your application that you visit, such as checking for accessibility.

One such check is automatically checking for JavaScript errors on page load. There’s a couple of approaches out there, one involves copying and pasting a small snippet of JavaScript into each page. Since our application we are working on uses a standard template for every page, we simply add some JavaScript to the common page header that is the first thing to load on the page, and catches any JavaScript errors that occur:

define(["amdUtils/string/interpolate"], function(interpolate) {
    window.jsErrors = [];

    window.onerror = function (errorMessage, url, lineNumber) {
        var message = interpolate("Error: [{{0}}], url: [{{1}}], line: [{{2}}]", [errorMessage, url, lineNumber]);
        return false;

It’s then a matter of checking the jsErrors each time we visit a page, this is example C# code we use in the base page class for every page.

var js = driver as IJavaScriptExecutor;
ICollection javascriptErrors = null;
for (var i = 0; i < 20; i++)
  javascriptErrors = js.ExecuteScript("return window.jsErrors") as ICollection;
  if (javascriptErrors != null) break;
Assert.IsNotNull(javascriptErrors, "Can't seem to load JavaScript on the page to find JavaScript errors. Check that JavaScript is enabled.");
var javaScriptErrorsAsString = javascriptErrors.Cast<string>().Aggregate("", (current, error) => current + (error + ", "));
Assert.AreEqual("", javaScriptErrorsAsString, "Found JavaScript errors on page load: " + javaScriptErrorsAsString);

This code waits until it can read the jsErrors on the page. If it can’t, it means that JavaScript didn’t load and this is an error. Once it gets the jsErrors, it checks that there are none.

This code has been very useful for us. It has caught a number of JavaScript errors, and is especially great for finding cross browser JavaScript issues, as we run our acceptance tests in 5 different browsers.

Twitter is over capacity

I like friendly error messages; I don’t think you can get much cuter than Twitter’s over capacity error message:

Twitter error message
Twitter error message

I just love how the whale is smiling. It makes it such a better error message. I don’t think many companies would be game to use a whale on their capacity error page (if they even had one) but in this case it just works.