Web frontend black-box casperjs tests

These live in frontend_tests/casper_tests/. This is a “black box” integration test; we load the frontend in a real (headless) browser, from a real (development) server, and simulate UI interactions like sending messages, narrowing, etc., by actually clicking around the UI and waiting for things to change before doing the next step. These tasks are fantastic for ensuring the overall health of the project, but are also costly to maintain and keep free of nondeterministic failures, so we usually prefer to write a Node test instead when possible.

Since the Casper tests interact with a real dev server, they can often catch backend bugs as well.

You can run the casper tests 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

When a Casper test fails, the first things to check (before you bother trying to use the Casper debugging tools are:

  • Does your branch actually work if you just open the webapp and try to follow the flow being tested? Often the answer is no, and you’ll find the debugging experience in your browser to be a much more convenient way to fix the issue.

  • Does your branch use ES6 syntax like arrow functions in a context that isn’t transpiled (i.e. non-TypeScript code)? Casper uses the PhantomJS browser, which doesn’t support ES6 syntax, so use of non-transpiled ES6 syntax will generally be first discovered via the Casper tests failing..

  • Are there any backend-errors (printed inline) while running the tests?

  • You can check the screenshots of what the UI looked like at the time of failures at var/casper/casper-failure*.png.

Remote debugging

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; you’ll likely also want to read the section on writing tests (below) if you get stuck, since the advice on how to write correct Casper selectors will likely be relevant.

This is a pain to setup with Vagrant because port 7777 and 9981 aren’t forwarded to the host by default, but can be pretty useful in rare difficult cases.

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.

Reproducing races only seen in Travis CI

We’ve sometimes found it useful for reproducing Casper race conditions in Casper tests that mostly only happen in Travis CI with really cheap VPS servers (e.g. Scaleway’s 2GB x86). This works because an ultra slow machine is more likely to have things happen in an order similar to what happens in Travis CI’s very slow containers.

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 waitUntilVisible, waitWhileVisible, or a similar function to make sure the page or element 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.waitUntilVisible("#btn-submit", function () {

    In many cases, you will actually need to wait for the UI to update clicking the button before doing asserts or the next step. This will ensure that the UI has finished updating from the previous step before Casper attempts to next step. The various wait functions supported in Casper are documented in the Casper here: https://web.archive.org/web/20200108100925if_/https://docs.casperjs.org/en/latest/modules/casper.html#waitforselector and the various assert statements available are documented here: https://web.archive.org/web/20190814204845if_/https://docs.casperjs.org/en/latest/modules/tester.html#the-tester-prototype

  • The casper.wait style functions (waitWhileVisible, waitUntilVisible, 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.waitUntilVisible('tag 1');
    casper.waitUntilVisible('tag 2');

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

    casper.then(function () {
        casper.waitUntilVisible('tag 1', function () {
    casper.then(function () {
        casper.waitUntilVisible('tag 2', function () {

    (You’ll also want to use selectors that are as explicit as possible, to avoid accidentally clicking multiple buttons or the wrong button in your test, which can cause nondeterministic failures)

  • Generally casper.waitUntilVisible is preferable to e.g. casper.waitForSelector, since the former will confirm the thing is actually on screen. E.g. if you’re waiting to switch from one panel of the the settings overlay to another by waiting for a particular widget to appear, casper.waitForSelector may not actually wait (since the widget is probably in the DOM, just not visible), but casper.waitUntilVisible will wait until it’s actually shown.

  • The selectors (i.e. things you put inside casper.waitUntilVisible() and friends) appearing in Casper tests are CSS3 selectors, which is a slightly different syntax from the jQuery selectors used in the rest of the Zulip codebase; in particular, some expressions that work with jQuery (and thus normal Zulip JavaScript code) won’t work with CSS3. It’s often helpful to debug selectors interactively, which you can do in the Chrome JavaScript console. The way to do it is $$("#settings-dropdown"); that queries CSS3 selectors, so you can debug your selector in the console and then paste it into your Casper test once it’s working. For other browsers like Firefox, you can use querySelectorAll("#settings-dropdown"), syntax which is 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: https://web.archive.org/web/20200107035425if_/https://docs.casperjs.org/en/latest/faq.html#how-does-then-and-the-step-stack-work