JavaScript/TypeScript unit tests

Our node-based unit tests system is the preferred way to test JavaScript/TypeScript code in Zulip. We prefer it over the Puppeteer black-box whole-app testing, system since it is much (>100x) faster and also easier to do correctly than the Puppeteer system.

You can run this test suite as follows:


See test-js-with-node --help for useful options; even though the whole suite is quite fast, it still saves time to run a single test by name when debugging something.

The JS unit tests are written to work with node. You can find them in frontend_tests/node_tests. Here is an example test from frontend_tests/node_tests/stream_data.js:

(function test_get_by_id() {
    var id = 42;
    var sub = {
        name: 'Denmark',
        subscribed: true,
        color: 'red',
        stream_id: id
    stream_data.add_sub('Denmark', sub);
    sub = stream_data.get_sub('Denmark');
    assert.equal(sub.color, 'red');
    sub = sub_store.get(id);
    assert.equal(sub.color, 'red');

The names of the node tests generally align with the names of the modules they test. If you modify a JS module in static/js you should see if there are corresponding test in frontend_tests/node_tests. If there are, you should strive to follow the patterns of the existing tests and add your own tests.

A good first test to read is example1.js. (And then there are several other example files.)

How the node tests work

Unlike the Puppeteer unit tests, which use a headless Chromium browser connected to a running Zulip development server, our node unit tests don’t have a browser, don’t talk to a server, and generally don’t use a complete virtual DOM (a handful of tests use the jsdom library for this purpose) because those slow down the tests a lot, and often don’t add much value.

Instead, the preferred model for our unit tests is to mock DOM manipulations (which in Zulip are almost exclusively done via jQuery) using a custom library zjquery.

The unit test file for zjquery is designed to be also serve as nice documentation for how to use zjquery, and is highly recommended reading for anyone working on or debugging the Zulip node tests.

Conceptually, the zjquery library provides minimal versions of most jQuery DOM manipulation functions, and has a convenient system for letting you set up return values for more complex functions. For example, if the code you’d like to test calls $obj.find(), you can use $obj.set_find_results(selector, $value) to set up zjquery so that calls to $obj.find(selector) will return $value. See the unit test file for details.

This process of substituting jQuery functions with our own code for testing purposes is known as “stubbing”. zjquery does not stub all possible interactions with the dom, as such, you may need to write out the stub for a function you’re calling in your patch. Typically the stub is just placed in the test file, to prevent bloating of zjquery with functions that are only used in a single test.

If you need to stub, you will see an error of this form: Error: You must create a stub for $("#foo").bar

The zjquery library itself is only about 500 lines of code, and can also be a useful resource if you’re having trouble debugging DOM access in the unit tests.

It is typically a good idea to figure out how to stub a given function based on how other functions have been stubbed in the same file.

Handling dependencies in unit tests

The other big challenge with doing unit tests for a JavaScript project is that often one wants to limit the scope the production code being run, just to avoid doing extra setup work that isn’t relevant to the code you’re trying to test. For that reason, each unit test file explicitly declares all of the modules it depends on, with a few different types of declarations depending on whether we want to:

  • Exercise the module’s real code for deeper, more realistic testing?

  • Stub out the module’s interface for more control, speed, and isolation?

  • Do some combination of the above?

For all the modules where you want to run actual code, add statements like the following toward the top of your test file:

zrequire('Filter', 'js/filter');

For modules that you want to completely stub out, use a pattern like this:

const reminder = mock_esm("../../static/js/reminder", {
    is_deferred_delivery: noop,

// then maybe further down
reminder.is_deferred_delivery = () => true;

One can similarly stub out functions in a module’s exported interface with either noop functions or actual code.

Finally, there’s the hybrid situation, where you want to borrow some of a module’s real functionality but stub out other pieces. Obviously, this is a pretty strong code smell that the other module might be lacking in cohesion, but sometimes it’s not worth going down the rabbit hole of trying to improve that. The pattern here is this:

// Import real code.

// And later... = function () {
    return 'office';

Creating new test modules

The test runner (index.js) automatically runs all .js files in the frontend_tests/node_tests directory, so you can simply start editing a file in that directory to create a new test.

Coverage reports

You can automatically generate coverage reports for the JavaScript unit tests like this:

    tools/test-js-with-node --coverage

If tests pass, you will get instructions to view coverage reports in your browser.

Note that modules that we don’t test at all aren’t listed in the report, so this tends to overstate how good our overall coverage is, but it’s accurate for individual files. You can also click a filename to see the specific statements and branches not tested. 100% branch coverage isn’t necessarily possible, but getting to at least 80% branch coverage is a good goal.

The overall project goal is to get to 100% node test coverage on all data/logic modules (UI modules are lower priority for unit testing).

Editor debugger integration

Our node test system is pretty simple, and it’s possible to configure the native debugger features of popular editors to allow stepping through the code. Below we document the editors where someone has put together detailed instructions for how to do so. Contributions of notes for other editors are welcome!

Webstorm integration setup

These instructions assume you’re using the Vagrant development environment.

  1. Set up Vagrant in WebStorm.

  2. In WebStorm, navigate to Preferences -> Tools -> Vagrant and configure the following:

    • Instance folder should be the root of the zulip repository on your host (where the Vagrantfile is located).

    • Provider should be virtualbox on macOS and Docker on Linux

    • In Boxes, choose the one used for Zulip (unless you use Virtualbox for other things, there should only be one option).

    You shouldn’t need to set these additional settings:

    • Vagrant executable should already be correctly vagrant.

    • Environment Variables is not needed.

  3. You’ll now need to set up a WebStorm “Debug Configuration”. Open the Run/Debug Configuration menu and create a new Node.js config:

    1. Under Node interpreter: click the 3 dots to the right side and click on the little plus in the bottom left of the Node.js Interpreters window.

    2. Select Add Remote....

      1. In the Configure Node.js Remote Interpreter, window select Vagrant

      2. Wait for WebStorm to connect to Vagrant. This will be displayed by the Vagrant Host URL section updating to contain the Vagrant SSH URL, e.g. ssh://vagrant@

      3. Set the Node.js interpreter path to /usr/local/bin/node

      4. Hit OK 2 times to get back to the Run/Debug Configurations window.

    3. Under Working Directory select the root zulip directory.

    4. Under JavaScript file, enter frontend_tests/zjsunit/index.js – this is the root script for Zulip’s node unit tests.

Congratulations! You’ve now set up the integration.

Running tests with the debugger

To use Webstorm to debug a given node test file, do the following:

  1. Under Application parameters choose the node test file that you are trying to test (e.g. frontend_tests/node_tests/message_store.js).

  2. Under Path Mappings, set Project Root to /srv/zulip (i.e. where the zulip Git repository is mounted in the Vagrant guest).

  3. Use the WebStorm debugger; see this overview for details on how to use it.