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Yoast SEO Development: Unit tests, code style and linters

At Yoast we use a variety of tools to ensure our code adheres to a certain set of standards, which allow us to ship our products with more confidence and less bugs.

These tools include the following:

  • Unit tests - To ensure isolated functions / methods behave as expected.
  • Integration tests - To ensure that a chain of actions (i.e function/method calls) behave as expected.
  • Linters - To ensure that code is written in a uniform way (i.e. structure, spacing etc.).


Unit tests

For our PHP code, we make use of PHPUnit to test code for expected behavior. Please make sure you've read the guide on setting up PHPUnit before running any tests.

Also make sure to read up on the section regarding running tests in a multisite environment.

Running unit test with code coverage

Running unit tests with code coverage is very similar to 'just' running unit tests, with one difference: While a test runs, all your code gets analyzed to see what parts of your code base are covered by the tests.

The results of this analysis are then stored as a report that can later be used to determine what parts of your code might need more tests. Please note that running unit tests with code coverage takes up more time, as analyzing your code requires extra resources.

To configure your system to be able to run with code coverage, follow these steps:

  1. Run pecl install xdebug in your terminal to install XDebug.
  2. To run the tests including the coverage, click Coverage button in PhpStorm in the upper right corner of PhpStorm.

XDebug won't load

If you get an error "Failed loading /usr/local/Cellar/php@7.1/7.1.26/lib/php/20160303/" or something similar, it is possible your extension directory needs to be configured so PHP knows where to look for it.

Follow these steps to configure your extension directory:

  1. Run php --ini in your terminal to find your php.ini file.
  2. Open the php.ini file in your preferred editor.
  3. To find out what the proper extension directory is, run: pecl config-get ext_dir in a separate terminal window.
  4. Now uncomment and update the extension_dir in your php.ini:extension_dir = /usr/local/lib/php/pecl/<php_api_version> and replace the pathname with the one you found in the previous step.

Sometimes the unit tests will run on a different PHP version than the one used for code coverage. This is because of a quirk in PhpStorm.

To fix this, under Edit Configurations... -> Command line -> Interpreter, you can specify an interpreter. However, running with code coverage always uses the <Default project interpreter>, even when you have specified another interpreter. In case you get fatal errors when running with code coverage, take a look at the default project interpreter (Preferences -> Languages & Frameworks -> PHP) and ensure that this one is also properly configured.

XDebug doesn't have the right SSL permissions

If you get an error regarding SSL permissions, please run brew install openssl and try to install XDebug again.

Integration tests

If you're interested in getting integration tests up and running, make sure to check out our guide on Setting up integration tests for the plugins.

Other plugins

Most of our other plugins also contain a variety of tests, written to check the PHP code. To run these tests, use the following command in your terminal:

  • composer test


Checking for PHP syntax errors


  • Use composer lint


  • Use composer premium-lint


To check for syntax errors, run find -L . -path ./vendor -prune -o -path ./node_modules -prune -o -name '*.php' -print0 | xargs -0 -n 1 -P 4 php -l

Coding standards

When using a linter on our code base, we utilize a combination of various coding standards, which are as follows:

Running the linter


  • For an interactive menu use: composer cs
  • To check the files of your branch against a branch other than trunk: composer check-branch-cs {BRANCH-TO-CHECK-AGAINST}. The parameter should be the name of the branch you wish to merge to.


  • All the commands from Free work for Premium as well.


  • To check everything: grunt phpcs


  • To check everything: vendor/bin/phpcs


  • To check everything: grunt phpcs


Unit tests

For our JavaScript code, we utilize Jest and Jasmine to test code for expected behavior. Please ensure you've run yarn prior to trying to run the following commands so that all necessary NPM packages are installed.

To run the tests, use the following command in your terminal:

  • yarn test


To run the tests, including coverage, use the following command in your terminal:

  • yarn test --coverage


Run linter automatically

To enable this automatic usage, select the Automatic ESlint configuration option in the settings by navigating to the following:

PhpStorm -> Preferences -> Editor -> Inspections -> JavaScript -> Code quality tools -> ESLint

Run linter manually

To run the linter, use the following command in your terminal:

  • grunt eslint


JavaScript tests