Agent Skills: Code Formatter

Code Formatter

Overview

Formats and validates code files using Prettier and related formatting tools. Supports JavaScript, TypeScript, JSON, CSS, Markdown, and many other file types.

Prerequisites

• Node.js (v16+) and npm/npx installed
• Prettier available globally (npm install -g prettier) or locally in the project
• Write permissions for target files and configuration directories
• Supported file types present in the project (.js, .jsx, .ts, .tsx, .json, .css, .md)

Instructions

1. Check whether Prettier is available by running npx prettier --version. If missing, install it locally with npm install --save-dev prettier or globally with npm install -g prettier.
2. Detect existing configuration by searching for .prettierrc, .prettierrc.json, prettier.config.js, or a "prettier" key in package.json. If no configuration exists, create a .prettierrc with sensible defaults (see ${CLAUDE_SKILL_DIR}/references/implementation.md).
3. Run npx prettier --check "**/*.{js,jsx,ts,tsx,json,css,md}" --ignore-path .prettierignore to identify files that need formatting. Report the count and list of non-conforming files.
4. Apply formatting to identified files using npx prettier --write on the target paths. For single files, specify the exact path; for directories, use glob patterns.
5. Create or update .prettierignore to exclude generated outputs (dist/, build/, *.min.js, *.min.css), dependencies (node_modules/, vendor/), and lock files.
6. Optionally set up pre-commit enforcement by installing husky and lint-staged, then configuring lint-staged in package.json to run prettier --write on staged files matching supported extensions.
7. Run a final npx prettier --check to confirm all target files now conform to the configured style rules.

Output

A formatting execution report containing:

• Count of files checked and files reformatted
• List of files that were modified with before/after formatting status
• Configuration file(s) created or updated (.prettierrc, .prettierignore)
• Any git hook integration changes applied
• Confirmation of final formatting compliance

Error Handling

| Error | Cause | Solution | |---|---|---| | prettier: command not found | Prettier not installed globally or locally | Run npm install -g prettier or npx prettier --version to use npx | | Syntax errors in source files | Malformed code that Prettier cannot parse | Fix syntax errors first using npx eslint --fix-dry-run <file> then retry formatting | | Configuration conflicts | Multiple .prettierrc files or conflicting editorconfig | Locate all config files with find . -name ".prettier*" and consolidate to a single config | | Permission denied on write | File or directory lacks write permission | Run chmod u+w <file> to grant write access | | Parser not found for file type | Unsupported file extension or missing Prettier plugin | Install the appropriate Prettier plugin (e.g., prettier-plugin-svelte) or exclude the file type |

Examples

Format a single file: Trigger: "Format src/app.js" Process: Run npx prettier --write src/app.js. Report the file as reformatted or already conformant.

Project-wide formatting setup: Trigger: "Set up code formatting for this project." Process: Create .prettierrc with project defaults, create .prettierignore excluding build outputs, run npx prettier --write "src/**/*.{js,ts,json,css}", install husky and lint-staged for pre-commit hooks, verify compliance.

Check formatting without modifying files: Trigger: "Check formatting across the project." Process: Run npx prettier --check "**/*.{js,jsx,ts,tsx,json,css,md}". Report non-conforming files with their paths. Suggest npx prettier --write to fix.

Resources

• ${CLAUDE_SKILL_DIR}/references/implementation.md -- detailed implementation guide with configuration examples
• ${CLAUDE_SKILL_DIR}/references/errors.md -- common error scenarios and solutions
• Prettier documentation -- official configuration and CLI reference
• ESLint -- complementary linting and code quality tool
• Husky -- git hooks for pre-commit formatting enforcement