JavaScript unit tests

As an alternative to the black-box whole-app testing, you can unit test individual JavaScript files.

If you are writing JavaScript code that manipulates data (as opposed to coordinating UI changes), then you probably modify existing unit test modules to ensure the quality of your code and prevent regressions.

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 = stream_data.get_sub_by_id(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.

HTML output

The JavaScript unit tests can generate output to be viewed in the browser. The best examples of this are in frontend_tests/node_tests/templates.js.

The main use case for this mechanism is to be able to unit test templates and see how they are rendered without the complications of the surrounding app. (Obviously, you still need to test the app itself!) The HTML output can also help to debug the unit tests.

Each test calls a method named write_handlebars_output after it renders a template with similar data. This API is still evolving, but you should be able to look at existing code for patterns.

When you run tools/test-js-with-node, it will present you with a message like “To see more output, open var/test-js-with-node/index.html.” Basically, you just need to open the file in the browser. (If you are running a VM, this might require switching to another terminal window to launch the open command.)

Coverage reports

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

tools/test-js-with-node cover

Then open coverage/lcov-report/js/index.html in your browser. Modules 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.

Handling dependencies in unit tests

The following scheme helps avoid tests leaking globals between each other.

First, if you can avoid globals, do it, and the code that is directly under test can simply be handled like this:

var search = require('js/search_suggestion.js');

For deeper dependencies, you want to categorize each module as follows:

  • 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 a statement like the following to the top of your test file:

    _: 'node_modules/underscore/underscore.js',
    util: 'js/util.js',
    Dict: 'js/dict.js',
    Handlebars: 'handlebars',
    Filter: 'js/filter.js',
    typeahead_helper: 'js/typeahead_helper.js',
    stream_data: 'js/stream_data.js',
    narrow: 'js/narrow.js'

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

set_global('page_params', {
    email: ''

// then maybe further down = '';

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 smell that the other module might be lacking in cohesion, but that code might be outside your jurisdiction. The pattern here is this:

// Use real versions of parse/unparse
var narrow = require('js/narrow.js');
set_global('narrow', {
    parse: narrow.parse,
    unparse: narrow.unparse

// But later, I want to stub the stream without having to call super-expensive
// real code like narrow.activate(). = function () {
    return 'office';

Creating new test modules

The nodes tests rely on JS files that use the module pattern. For example, to test the foobar.js file, you would first add the following to the bottom of foobar.js:

if (typeof module !== 'undefined') {
    module.exports = foobar;

This makes foobar.js follow the CommonJS module pattern, so it can be required in Node.js, which runs our tests.

Now create frontend_tests/node_tests/foobar.js. At the top, require the Node.js assert module, and the module you’re testing, like so:

var assert = require('assert');
var foobar = require('js/foobar.js');

And of course, if the module you’re testing depends on other modules, or modifies global state, you may need to review the section on handling dependencies above.

Define and call some tests using the assert module. Note that for “equal” asserts, the actual value comes first, the expected value second.

(function test_somefeature() {
    assert.strictEqual(foobar.somefeature('baz'), 'quux');
    assert.throws(foobar.somefeature('Invalid Input'));

The test runner (index.js) automatically runs all .js files in the frontend_tests/node directory.