Style guides

Editor/IDE styling standardization

We use EditorConfig to automatically apply certain styling standards before files are saved locally. Most editors/IDEs will honor the .editorconfig settings automatically by default. If your editor/IDE does not automatically support .editorconfig, we suggest investigating to see if a plugin exists. For instance here is the plugin for vim.

Pre-push static analysis with Lefthook

Lefthook is a Git hooks manager that allows custom logic to be executed prior to Git committing or pushing. GitLab comes with Lefthook configuration (lefthook.yml), but it must be installed.

We have a lefthook.yml checked in but it is ignored until Lefthook is installed.

Uninstall Overcommit

We were using Overcommit prior to Lefthook, so you may want to uninstall it first with overcommit --uninstall.

Install Lefthook

  1. Install the lefthook Ruby gem:

    bundle install
  2. Install Lefthook managed Git hooks:

    bundle exec lefthook install
  3. Test Lefthook is working by running the Lefthook prepare-commit-msg Git hook:

    bundle exec lefthook run prepare-commit-msg

This should return a fully qualified path command with no other output.

Lefthook configuration

The current Lefthook configuration can be found in lefthook.yml.

Before you push your changes, Lefthook automatically runs the following checks:

  • Danger: Runs a subset of checks that danger-review runs on your merge requests.
  • ES lint: Run yarn run internal:eslint checks (with the .eslintrc.yml configuration) on the modified *.{js,vue} files. Tags: frontend, style.
  • HAML lint: Run bundle exec haml-lint checks (with the .haml-lint.yml configuration) on the modified *.html.haml files. Tags: view, haml, style.
  • Markdown lint: Run yarn markdownlint checks on the modified *.md files. Tags: documentation, style.
  • SCSS lint: Run yarn lint:stylelint checks (with the .stylelintrc configuration) on the modified *.scss{,.css} files. Tags: stylesheet, css, style.
  • RuboCop: Run bundle exec rubocop checks (with the .rubocop.yml configuration) on the modified *.rb files. Tags: backend, style.
  • Vale: Run vale checks (with the .vale.ini configuration) on the modified *.md files. Tags: documentation, style.

In addition to the default configuration, you can define a local configuration.

Disable Lefthook temporarily

To disable Lefthook temporarily, you can set the LEFTHOOK environment variable to 0. For instance:

LEFTHOOK=0 git push ...

Run Lefthook hooks manually

To run the pre-push Git hook, run:

bundle exec lefthook run pre-push

For more information, check out Lefthook documentation.

Skip Lefthook checks per tag

To skip some checks based on tags when pushing, you can set the LEFTHOOK_EXCLUDE environment variable. For instance:

LEFTHOOK_EXCLUDE=frontend,documentation git push ...

For more information, check out Lefthook documentation.

Ruby, Rails, RSpec

Our codebase style is defined and enforced by RuboCop.

You can check for any offenses locally with bundle exec rubocop --parallel. On the CI, this is automatically checked by the static-analysis jobs.

For RuboCop rules that we have not taken a decision on yet, we follow the Ruby Style Guide, Rails Style Guide, and RSpec Style Guide as general guidelines to write idiomatic Ruby/Rails/RSpec, but reviewers/maintainers should be tolerant and not too pedantic about style.

Similarly, some RuboCop rules are currently disabled, and for those, reviewers/maintainers must not ask authors to use one style or the other, as both are accepted. This isn’t an ideal situation since this leaves space for bike-shedding, and ideally we should enable all RuboCop rules to avoid style-related discussions/nitpicking/back-and-forth in reviews.

Additionally, we have a dedicated newlines style guide, as well as dedicated test-specific style guides and best practices.

Creating new RuboCop cops

Typically it is better for the linting rules to be enforced programmatically as it reduces the aforementioned bike-shedding.

To that end, we encourage creation of new RuboCop rules in the codebase.

When creating a new cop that could be applied to multiple applications, we encourage you to add it to our GitLab Styles gem.

Resolving RuboCop exceptions

When the number of RuboCop exceptions exceed the default exclude-limit of 15, we may want to resolve exceptions over multiple commits. To minimize confusion, we should track our progress through the exception list.

When auto-generating the .rubocop_todo.yml exception list for a particular Cop, and more than 15 files are affected, we should add the exception list to a different file, .rubocop_manual_todo.yml.

This ensures that our list isn’t mistakenly removed by another auto generation of the .rubocop_todo.yml. This also allows us greater visibility into the exceptions which are currently being resolved.

One way to generate the initial list is to run the todo auto generation, with exclude limit set to a high number.

bundle exec rubocop --auto-gen-config --auto-gen-only-exclude --exclude-limit=100000

You can then move the list from the freshly generated .rubocop_todo.yml for the Cop being actively resolved and place it in the .rubocop_manual_todo.yml. In this scenario, do not commit auto generated changes to the .rubocop_todo.yml as an exclude limit that is higher than 15 will make the .rubocop_todo.yml hard to parse.

Reveal existing RuboCop exceptions

To reveal existing RuboCop exceptions in the code that have been excluded via .rubocop_todo.yml and .rubocop_manual_todo.yml, set the environment variable REVEAL_RUBOCOP_TODO to 1.

This allows you to reveal existing RuboCop exceptions during your daily work cycle and fix them along the way.

notePermanent Excludes should be defined in .rubocop.yml instead of .rubocop_manual_todo.yml.

Database migrations

See the dedicated Database Migrations Style Guide.


See the dedicated JS Style Guide.


See the dedicated SCSS Style Guide.


See the dedicated Go standards and style guidelines.

Shell commands (Ruby)

See the dedicated Guidelines for shell commands in the GitLab codebase.

Shell scripting

See the dedicated Shell scripting standards and style guidelines.


We’re following Ciro Santilli’s Markdown Style Guide.


See the dedicated Documentation Style Guide.


See the dedicated Python Development Guidelines.


Code should be written in US English.

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