Web frontend black-box casperjs tests

These live in frontend_tests/casper_tests/. This is a “black box” test; we load the frontend in a real (headless) browser, from a real (development) server, and simulate UI interactions like sending messages, narrowing, etc.

Since this is interacting with a real dev server, it can catch backend bugs as well.

You can run this with ./tools/test-js-with-casper or as ./tools/test-js-with-casper 06-settings.js to run a single test file from frontend_tests/casper_tests/.

Debugging Casper.JS

Casper.js (via PhantomJS) has support for remote debugging. However, it is not perfect. Here are some steps for using it and gotchas you might want to know.

To turn on remote debugging, pass --remote-debug to the ./frontend_tests/run-casper script. This will run the tests with port 7777 open for remote debugging. You can now connect to localhost:7777 in a Webkit browser. Somewhat recent versions of Chrome or Safari might be required.

  • When connecting to the remote debugger, you will see a list of pages, probably 2. One page called about:blank is the headless page in which the CasperJS test itself is actually running in. This is where your test code is.
  • The other page, probably localhost:9981, is the Zulip page that the test is testing—that is, the page running our app that our test is exercising.

Since the tests are now running, you can open the about:blank page, switch to the Scripts tab, and open the running 0x-foo.js test. If you set a breakpoint and it is hit, the inspector will pause and you can do your normal JS debugging. You can also put breakpoints in the Zulip webpage itself if you wish to inspect the state of the Zulip frontend.

You can also check the screenshots of failed tests at /tmp/casper-failure*.png.

If you need to use print debugging in casper, you can do using casper.log; see http://docs.casperjs.org/en/latest/logging.html for details.

An additional debugging technique is to enable verbose mode in the Casper tests; you can do this by adding to the top of the relevant test file the following:

var casper = require('casper').create({
   verbose: true,
   logLevel: "debug"

This can sometimes give insight into exactly what’s happening.

Writing Casper tests

Probably the easiest way to learn how to write Casper tests is to study some of the existing test files. There are a few tips that can be useful for writing Casper tests in addition to the debugging notes below:

  • Run just the file containing your new tests as described above to have a fast debugging cycle.

  • With frontend tests in general, it’s very important to write your code to wait for the right events. Before essentially every action you take on the page, you’ll want to use waitForSelector, waitUntilVisible, or a similar function to make sure the page or elemant is ready before you interact with it. For instance, if you want to click a button that you can select via #btn-submit, and then check that it causes success-elt to appear, you’ll want to write something like:

    casper.waitForSelector("#btn-submit", function () {

    This will ensure that the element is present before the interaction is attempted. The various wait functions supported in Casper are documented in the Casper here: http://docs.casperjs.org/en/latest/modules/casper.html#waitforselector and the various assert statements available are documented here: http://docs.casperjs.org/en/latest/modules/tester.html#the-tester-prototype

  • The ‘waitFor’ style functions (waitForSelector, etc.) cannot be chained together in certain conditions without creating race conditions where the test may fail nondeterministically. For example, don’t do this:

      casper.waitForSelector('tag 1');
      casper.waitForSelector('tag 2');

    Instead, if you want to avoid race condition, wrap the second waitFor in a then function like this:

      casper.waitForSelector('tag 1');
      casper.then(function () {
          casper.waitForSelector('tag 2');
  • The selectors appearing in Casper tests are CSS3 selectors, which is a slightly different syntax from jQuery selectors. You can often save time by testing and debugging your selectors on the relevant page of the Zulip development app in the Chrome JavaScript console by using e.g. $$("#settings-dropdown") (this syntax is the CSS selector equivalent to querySelectorAll(), only available in the browser’s JavaScript console).

    You can learn more about these selectors and other JavaScript console tools here.

  • The test suite uses a smaller set of default user accounts and other data initialized in the database than the development environment; to see what differs check out the section related to options["test_suite"] in zilencer/management/commands/populate_db.py.

  • Casper effectively runs your test file in two phases – first it runs the code in the test file, which for most test files will just collect a series of steps (each being a casper.then or casper.wait... call). Then, usually at the end of the test file, you’ll have a casper.run call which actually runs that series of steps. This means that if you write code in your test file outside a casper.then or casper.wait... method, it will actually run before all the Casper test steps that are declared in the file, which can lead to confusing failures where the new code you write in between two casper.then blocks actually runs before either of them. See this for more details about how Casper works: http://docs.casperjs.org/en/latest/faq.html#how-does-then-and-the-step-stack-work