Welcome to Mastering Frontend, an ongoing series of cookbook-style posts for advanced frontend engineering. You will learn modern best practices, advanced features, and ways to write, measure, and maintain high quality code for ambitious frontend applications. Presented using Ember.js, the tools and techniques you’ll learn apply to most frontend frameworks.

Maintainability Test Coverage

There are many ways to measure “code quality”, and no single one is “right”. Tools, techniques, and opinions are always evolving. It’s a good idea to instrument for several quality metrics to get a well-rounded view of your code. Use them as guides without optimizing for them too religiously.

This post’s recommendations are exemplified in its repository. Builds for maintainability and test coverage may be viewed on Code Climate.


ESLint is a linting tool for JavaScript that detects code errors, common faux pas, and can enforce some stylistic guidelines. Ember apps ship with ESLint: each JavaScript file is linted automatically as a separate test case. Linting is a first line of defense in the maintenance of quality code. While some sensible default linting rules are applied for you, it’s worth learning more about the capabilities of ESLint.

Many modern frontend frameworks come with linting enabled by default. When it’s not the default, it’s easy to setup. I recommend linting even the smallest of projects.

Test Coverage

I’ll assume you already write tests as a habit. But how do you know that you have enough of the right tests? Fortunately, it’s easy to measure the amount of code tested. This is called test coverage. It’s a simple accounting of the code that executes when tests run. Anything less than 100% coverage means that some code isn’t executed during tests.

To get started in Ember, install the coverage add-on:

ember install ember-cli-code-coverage

Update the test task in package.json to enable coverage:
"test": "COVERAGE=true ember test".

Create a config/coverage.js file:

'use strict';

module.exports = {
  reporters: ['lcov', 'html', 'text-summary']

That’s it! When tests run, you should see a summary report similar to the following:

======== Coverage summary ========
Statements   : 100% ( 4/4 )
Branches     : 100% ( 0/0 )
Functions    : 0% ( 0/1 )
Lines        : 100% ( 4/4 )

You also get a convenient HTML report in coverage/lcov-report/index.html. This breaks down coverage line-by-line, file-by-file, and highlights code that was missed.

Test Parallelization & Randomization

Ember Exam is an add-on that enhances test execution with parallelization and order randomization. Parallelization is useful for running tests more quickly. Test order randomization helps to prevent subtle errors from slipping through unnoticed, which can happen if tests inadvertently become order-dependent.

To get started, install the add-on:

ember install ember-exam

Now update the test task in package.json to use Ember Exam:
"test": "COVERAGE=true ember exam --split 2 --parallel --random"

Code Climate

Code Climate is a service that monitors code quality. By default, it assesses structure and duplication. Many other options are available.

To get started, login to GitHub and then visit Code Climate to login with one click via GitHub. Once you’re in, enable the desired repo(s). The first run of Code Climate may take a few seconds or minutes, depending on your code. When it’s done its evaluation, you’ll get letter scores for your repo and individual files.

Code Climate is also great for monitoring code coverage. The test coverage we setup earlier can be sent to Code Climate:

  • Navigate to the settings page for your repo in Code Climate.
  • Copy the “test reporter ID”.
  • Add the value to the CC_TEST_REPORTER_ID environment variable in Travis.
  • Add coverage reporting steps to .travis.yml:
  - curl -L https://codeclimate.com/downloads/test-reporter/test-reporter-latest-linux-amd64 > ./cc-test-reporter
  - chmod +x ./cc-test-reporter
  - ./cc-test-reporter before-build
  - ./cc-test-reporter after-build --exit-code $TRAVIS_TEST_RESULT

Commit and push.

More Badges

Congratulations. You’ve earned two new badges: one for maintainability and one for test coverage (the badges for this post’s example repo are shown above). You can find code for these badges on the settings page for your repo in Code Climate. Drop them into your readme, commit, and push.

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