We want to maintain a project with a high coverage (aiming for 100%). Our main coverage criterion is branch coverage, that is making sure all possible code branches are covered by test. At an absolute bare minimum, we require 100% line coverage. Codecov monitors our test coverage for every Pull Request. You can make sure you haven't decreased coverage by checking the Pull Request on GitHub. You can also generate a coverage report locally.

General guidelines

  • Imports should be alphabetized, even in test files.
  • Comments should be full sentences to ease readability.
  • There are a lot of helpers in the tests/unit/helpers and tests/unit/amo/helpers modules–please use them.
  • Use action creators in amo/actions/ and core/actions/ instead of hard-coding dispatch() arguments or state data for tests. This applies to both UI components and reducers/sagas.
  • Use constants (see core/constants) when using the same value across files. This avoids hard-coding values and magic constants.


Jest is our main testing framework. Please refer to the README section about running the test suite to know how to run the tests. Below are a few rules regarding Jest:

  • Prefer toEqual() over toBe().
  • Do not use expect.assertions(N), this is hard to maintain. Instead, add a catch() branch to detect unexpected errors.

When creating a new test file, start with a describe() block that takes the current file name as first argument. This makes easy to find/edit a failing test case as Jest will display the test file in its output:

describe(__filename, () => {});

Spies/Stubs/Mocks and Sinon.JS

We use sinon for spies, stubs and mocks. In addition, we use sinon assertions over Jest expectations because failure messages are more descriptive.

const spy = sinon.spy();
// ...



There is no need to import sinon, it is already in the global scope for all test files.

Testing reducers and sagas

For sagas/reducers, there are two useful helpers: dispatchClientMetadata() and dispatchSignInActions() (tests/unit/amo/helpers) that should be used to initialize state in a realistic manner. The former is used to obtain a non-authenticated state while the latter returns an authenticated state.

When you need a errorHandler or a errorHandlerId, use the createStubErrorHandler() helper from tests/unit/helpers.

When asserting for exceptions/errors, do not use ES6 shorthand functions/implicit return functions. Instead, you should make visible the method/function that should thrown an exception:

expect(() => {
}).toThrow(/expected error message/);

When testing sagas, use an action creator to construct the expected actions that should be called by the saga under test:

const expectedLoadAction = autocompleteLoad(results);

await sagaTester.waitFor(expectedLoadAction.type);

const loadAction = sagaTester.getCalledActions()[2];

Testing UI components

We use Enzyme for testing UI components (React components). You should test the final component ideally. Below are a few rules regarding Enzyme:

  • Prefer shallow() over mount() when it makes sense.
  • Assert components on public properties (props), e.g.:

js expect(root.find(Badge)).toHaveProp('type', 'experimental');

  • You can use shallowUntilTarget() (tests/unit/helpers) for testing a component wrapped in one or more HOCs (higher order components). See tests/unit/core/components/TestInstallButton.js for an example of a test case with this helper.