ci: add prettier linting

[MikeMcC399]

Status

JavaScript files are linted for stylistic formatting using @stylistic/eslint-plugin.

Other types of files are not linted for formatting.

Assessment

Prettier is used in some other Cypress repos to provide consistent code formatting, in addition to @stylistic/eslint-plugin. It supports several file types including the following used in this repo:

• JavaScript
• JSON
• Markdown
• YAML

JavaScript formatting is already the responsibility of @stylistic/eslint-plugin in this repo. Prettier has a different opinion about JavaScript and the two cannot be used together. Cypress itself does not use Prettier to format code - see CONTRIBUTING > Coding Style.

Change

• Implement Prettier source file formatting with [email protected]
• Target JSON and YAML initially
• Defer linting Markdown until a follow-on PR
• Exclude JavaScript and TypeScript from linting by Prettier
• Add scripts to package.json

  "format": "prettier --write .",
  "format:check": "prettier --check .",

• Add instructions to the CONTRIBUTING document to run format
• Add format:check to circle.yml pipeline lint job and to .husky/pre-commit

Verification

On Ubuntu 24.04.2 LTS, Node.js 22.16.0 LTS

Execute

git clean -xfd
npm ci
npm run format:check

and confirm

All matched files use Prettier code style!

Execute

npm run format

and confirm that only the following file types are linted by Prettier at this time. Each should also be annotated with (unchanged):

• *.yml
• *.json

[cypress-app-bot]

• Create a Draft Pull Request if your PR is not ready for review. Mark the PR as Ready for Review when you're ready for a Cypress team member to review the PR.