Day 3 – Linting Testing and Tooling

Today's Focus

Set up ESLint, Prettier, and a test runner; write unit tests for your TypeScript domain logic.

Tasks

• Install and configure ESLint for TypeScript: npm install --save-dev eslint @typescript-eslint/parser @typescript-eslint/eslint-plugin. Create .eslintrc.json with @typescript-eslint/recommended rules. Run it and fix every reported error and warning.
• Install Prettier: npm install --save-dev prettier. Create .prettierrc with your preferences. Add a .prettierignore for dist/. Add npm run format (write) and npm run format:check (ci check) scripts.
• Set up a test runner: install vitest (or jest with ts-jest). Write at least 8 unit tests for your domain models and the Result type from Tuesday. Test both the happy path and error branches.
• Add npm run lint, npm run test, and npm run test:coverage scripts. Configure vitest to generate a coverage report and aim for 80% line coverage.
• Add a .editorconfig file to enforce consistent indentation and line endings across editors. Verify VS Code respects it.
• Create a pre-commit hook using husky and lint-staged that runs eslint and prettier --check on staged .ts files only. Commit a deliberately malformed file to confirm the hook blocks it.

Reading / Reference

• typescript-eslint getting started.
• Vitest documentation — Getting Started and Features.
• Prettier documentation — Installation and Integrating with Linters.