Write Tests Like a Mathematician: Part 1

Mathematical proofs can teach us how to write tests

Ember gives you the power to write tests and be productive from day one. You can be confident that your app will be correct today and years from now. A question remains: How should you write tests?

Since tests are a core part of the Ember framework and your development cycle, I will dedicate several blog posts on best practices for writing tests, based on my experience at work and former life as mathematician.

Today, we will cover why testing is important, what tools can help you with testing, and how to run and debug your tests.

Please note that some tools may be readily available for Ember only. However, the best practices that I will mention should be independent of your framework. After all, tests are a universal language, just like math is.

1. Why Do I Need Tests?

Writing tests is a necessary ingredient if you want to guarantee users and stakeholders that your app, whether small or large, will function as intended at any given time. The larger your app, the more costly and error-prone manual testing becomes.

xkcd comic takes a light jab at an untested method.
xkcd takes a light jab at an untested method. (source)

Writing tests is also a fun activity, a nice change of pace from delivering features daily, and a way to help you refactor code and improve as a developer. Tests can also serve as a living documentation—a key element in onboarding new developers.

2. What Tools Can Help Me?

a. QUnit, QUnit DOM

Every Ember app comes with QUnit and QUnit DOM. QUnit is a testing framework, and QUnit DOM is a library that helps you write tests that are concise and readable. You can use QUnit and QUnit DOM with any JavaScript framework, not just Ember.

To see the power of QUnit DOM, consider this code snippet. It checks whether our button component shows the right text and the right CSS attribute.

    For simplicity, the import, module, and setup statements
    are omitted here. Our component accepts two attributes,
    text (string) and isDisabled (boolean).
test('should show text', async function(assert) {
    await render(hbs`<SimpleButton @text="Hello world!" />`);

    // QUnit
    assert.strictEqual(this.element.textContent.trim(), 'Hello world!');

    // QUnit DOM
    assert.dom(this.element).hasText('Hello world!');

test('should allow disabling the button', async function(assert) {
    await render(hbs`<SimpleButton @text="Hello world!" @isDisabled={{true}} />`);

    // QUnit

    // QUnit DOM

b. Mocha, Chai DOM

Mocha is another testing framework. If you are more familiar with Mocha, you can install ember-mocha and Chai DOM instead.

Again, you can use Mocha and Chai DOM with any JavaScript framework. If you are using Ember, keep in mind that the setup functions from ember-qunitsetupTest, setupRenderingTest, and setupApplicationTest—need to be replaced with those from ember-mocha.

c. Ember CLI

When you use Ember CLI to generate an Ember “object” (e.g. component, model, service), it will create a test file with a setup that correctly addresses your testing framework and the type of test that you should write.

You can also use Ember CLI to create the test file separately from the object. For example, if you enter the following lines in the terminal,

ember g model-test student
ember g component-test student
ember g acceptance-test students

you get a unit test for the student model, a rendering test (integration test) for the student component, and an application test (acceptance test) that can be used to check the students route and its subroutes.

d. Ember Test Selectors

You want to be able to grab DOM elements in your tests. Since Ember is just JavaScript, you can use querySelector and querySelectorAll to do so. These methods require you to pass a selector, a string that identifies the element(s) that you want.

While you can use CSS classes as selectors, a best practice for testing is to separate the concerns between styling and testing. Class names and DOM structure change over time—for the better—by you, your team, and addon developers. If you rely on CSS classes, your tests will break and need a significant rewrite.

Ember Test Selectors is an addon that helps you write tests that are more resilient to DOM changes. You use data-test-* attributes to mark the elements that will be used in your tests. The addon works with QUnit DOM and helpers from @ember/test-helpers. It also removes the data-test-* attributes in the production build.

Consider the example of a button component again. This time, our component can display a Material icon in addition to the text.

<button type="button">
    {{#if icon}}
        <i data-test-icon class="material-icons">{{icon}}</i>

    <span data-test-text>{{text}}</span>

test('should show icon and text', async function(assert) {
    await render(hbs`<SimpleButton @icon="face" @text="Hello world!" />`);

    // Bad
        'The user sees the correct icon.'

        'Hello world!',
        'The user sees the correct text.'

    // Good
        'The user sees the correct icon.'

        'Hello world!',
        'The user sees the correct text.'

    // Great!
        .hasText('face', 'The user sees the correct icon.');

        .hasText('Hello world!', 'The user sees the correct text.');

e. Ember CLI Mirage

If your application receives and sends data, you want to show that you can take these actions successfully. You also want to prove that you can handle the error states correctly.

Ember CLI Mirage is an addon that allows you to create a mock server. You can also use it to test your app against various server states. To learn more about using Mirage in tests, we encourage you to visit the official website.

f. Ember Exam

You want your tests to finish fast. A fast run means you get to try out a different solution and iterate many more times.

Ember Exam is an addon that allows you to parallelize the run. If you have many rendering and application tests, this can dramatically speed up your testing.

Ember Exam allows you to parallelize and randomize tests.
Ember Exam allows you to parallelize and randomize tests.

Ember Exam also lets you randomize how the tests are run. Why would you want to do so? When you don’t properly set up and tear down a test, you can create dependencies among tests. Randomizing the order helps you catch these inadvertent bugs.

g. Percy

Last but not least, Percy is a visual regression testing tool that helps you catch accidental style changes. You can try it out for free and pay for additional service.

Percy's visual diff allows you to catch accidental style changes.
Percy’s visual diff allows you to catch accidental style changes.

While we don’t recommend this practice in general, you might also use Percy in lieu of application tests to capture complex workflows.

3. How to Run Tests

You have a few options for running tests.

First, you can run the test suite by entering the command ember test, or ember t, in your terminal. This will run the suite just once.

Suppose, instead, you want the suite to run after every file change. You can enter ember test --server, or ember t -s.

Lastly, if you are already running a local development server (through ember server), you can visit the /tests URI. This will render the tests/index.html template.

# Run all tests once
ember test
ember t

# Run all tests after every file change
ember test --server
ember t -s

a. How to Filter Tests

When you are working on a single component or page, you will want only a small subset of tests to run after every file change. To specify which tests to run, you can add --module or --filter option to your command.

The --module option allows you to select a module—a group of tests that you specified in module() in QUnit, or describe() in Mocha.

# Button component example
ember test --server --module="Integration | Component | simple-button"

# Run tests for a location service
ember t -s -m="Unit | Service | location"

The --filter option is more versatile. You can provide a phrase to match against the modules and test descriptions. A test description is what appears in test() in QUnit, or it() in Mocha.

# Button component example
ember test --server --filter="should show icon and text"

# Test everything related to your dashboard
ember t -s -f="dashboard"

# Run integration tests
ember t -s -f="integration"

With QUnit, you can also exclude tests by adding an exclamation point to the beginning of the filter, e.g. ember test --filter="!acceptance".

To learn more about options for testing, you can visit Ember CLI Documentation or type ember help test in the command line.

4. How to Debug Tests

When you are writing tests or application code, the execution of your tests may fail.

To find out the problem, you can add debugger to your code to check the intermediate state. You can add this line to both test and application code.

Thanks to Ember’s setup, you can also use pauseTest() and resumeTest() to debug your tests. pauseTest allows you to inspect the DOM easily, but can only be used in the test code.

Simply add await pauseTest(); to your test code, then save. When the test reaches this line, it will pause, allowing you to inspect the state of your application. When you are done, type resumeTest() in the browser console to continue the test.

You can use pauseTest to inspect the DOM.
You can use pauseTest to inspect the DOM.
Type resumeTest in the console when you are finished with debugging your test.
Type resumeTest when you are finished with debugging your test.

5. Summary

Ember considers testing a first-class citizen. (To my knowledge, it is the only JavaScript framework that covers testing in its official tutorial.) In addition to providing easy paths to integrate QUnit and Mocha, Ember supports a variety of addons and debugging tools to improve your developer experience in testing.

Next time, we will study 3 types of tests that Ember supports—unit, rendering, and application tests. We will look at each type and when you might use one over another.