Unit testing


Our unit tests in Activity Stream are written with mocha, chai, and sinon, and run with karma. They include unit tests for both content code (React components, etc.) and .sys.mjss.

You can find unit tests in tests/unit.


To run the unit tests once, execute npm test.

To run unit tests continuously (i.e. in “test-driven development” mode), you can run npm run tddmc.


To debug tests, you should run them in continuous mode with npm run tddmc. In the Firefox window that is opened (it should say “Karma… - connected”), click the “debug” button and open your console to see test output, set breakpoints, etc.

Unfortunately, source maps for tests do not currently work in Firefox. If you need to see line numbers, you can run the tests with Chrome by running npm install --save-dev karma-chrome-launcher && npm run tddmc -- --browsers Chrome

Where to put new tests

If you are creating a new test, add it to a subdirectory of the tests/unit that corresponds to the file you are testing. Tests should end with .test.js or .test.jsx if the test includes any jsx.

For example, if the file you are testing is lib/Foo.sys.mjs, the test file should be test/unit/lib/Foo.test.js

Mocha tests

All our unit tests are written with mocha, which injects globals like describe, it, beforeEach, and others. It can be used to write synchronous or asynchronous tests:

describe("FooModule", () => {
  // A synchronous test
  it("should create an instance", () => {
    assert.instanceOf(new FooModule(), FooModule);
  describe("#meaningOfLife", () => {
    // An asynchronous test
    it("should eventually get the meaning of life", async () => {
      const foo = new FooModule();
      const result = await foo.meaningOfLife();
      assert.equal(result, 42);


To write assertions, use the globally available assert object (this is provided by karma-chai, so you do not need to require it).

For example:

assert.equal(foo, 3);
assert.propertyVal(someObj, "foo", 3);

You can use any of the assertions from:

Custom assertions

We have some custom assertions for checking various types of actions:


Asserts that a given action is a valid User Event, i.e. that it contains only expected/valid properties for User Events in Activity Stream.

// This will pass
assert.isUserEventAction(ac.UserEvent({event: "CLICK"}));

// This will fail
assert.isUserEventAction({type: "FOO"});

// This will fail because BLOOP is not a valid event type
assert.isUserEventAction(ac.UserEvent({event: "BLOOP"}));

Overriding globals in .sys.mjss

Most .sys.mjss you will be testing use Cu.import or XPCOMUtils to inject globals. In order to add mocks/stubs/fakes for these globals, you should use the GlobalOverrider utility in test/unit/utils:

const {GlobalOverrider} = require("test/unit/utils");
describe("MyModule", () => {
  let globals;
  let sandbox;
  beforeEach(() => {
    globals = new GlobalOverrider();
    sandbox = globals.sandbox; // this is a sinon sandbox
    // This will inject a "AboutNewTab" global before each test
    globals.set("AboutNewTab", {override: sandbox.stub()});
  // globals.restore() clears any globals you added as well as the sinon sandbox
  afterEach(() => globals.restore());

Testing React components

You should use the enzyme suite of test utilities to test React Components for Activity Stream.

Where possible, use the shallow rendering method (this will avoid unnecessarily rendering child components):

const React = require("react");
const {shallow} = require("enzyme");

describe("<Foo>", () => {
  it("should be hidden by default", () => {
    const wrapper = shallow(<Foo />);

If you need to, you can also do Full DOM rendering with enzyme’s mount utility.

const React = require("react");
const {mount} = require("enzyme");
const wrapper = mount(<Foo />);