How to enforce code quality in Replit

Open the Shell pane from the Tools dock on the left sidebar. Run the npm command to install ESLint as a development dependency. ESLint is the industry-standard linter for JavaScript and TypeScript, used by the React, Next.js, and Node.js ecosystems. Installing it as a dev dependency keeps it available in the workspace without bloating your production deployment.

1npm install --save-dev eslint @eslint/js

ESLint needs a configuration file to know which rules to apply. Create an eslint.config.js file in your project root. The flat config format (introduced in ESLint v9) uses a simple JavaScript array of rule objects. Start with the recommended rule set, which catches the most common JavaScript errors like unused variables, unreachable code, and missing semicolons. You can customize rules later as your project's needs evolve.

1import js from "@eslint/js";
2
3export default [
4  js.configs.recommended,
5  {
6    rules: {
7      "no-unused-vars": "warn",
8      "no-console": "warn",
9      "eqeqeq": "error",
10      "no-var": "error"
11    }
12  }
13];

With ESLint installed and configured, run it against your source files from the Shell. The npx command runs the locally installed version of ESLint. Point it at your source directory or specific files. ESLint reports errors (must fix) and warnings (should fix) with the file name, line number, and a description of the issue. Fix the reported issues in your code, then run ESLint again to verify they are resolved.

1npx eslint src/
2
3# Or lint a specific file
4npx eslint src/App.jsx
5
6# Auto-fix what ESLint can fix automatically
7npx eslint src/ --fix

For Python projects, Pylint is the most comprehensive linter available. Install it with pip in the Shell. Pylint checks for errors, enforces coding standards (PEP 8), detects code smells, and even rates your code on a scale of 0 to 10. It works out of the box with sensible defaults, but you can customize its behavior with a .pylintrc file or pyproject.toml configuration.

1pip install pylint
2
3# Run Pylint on a specific file
4pylint main.py
5
6# Run Pylint on all Python files in a directory
7pylint src/

Create a .pylintrc file in your project root to customize which rules Pylint enforces. This is useful for silencing rules that do not apply to your project or adjusting severity levels. You can generate a default configuration with pylint --generate-rcfile, then edit it to match your preferences. Common customizations include disabling the missing-docstring warning for small projects and setting the maximum line length.

1[MAIN]
2max-line-length = 120
3
4[MESSAGES CONTROL]
5disable =
6    C0114,  # missing-module-docstring
7    C0115,  # missing-class-docstring
8    C0116,  # missing-function-docstring
9    R0903   # too-few-public-methods

For JavaScript projects, add a lint script to your package.json so the whole team uses the same command. For Python projects, you can add the lint command to the .replit file's run configuration or create a simple shell script. This standardizes linting across all collaborators and makes it easy to run before committing code or deploying. Consider running linting as the first step before each deployment to catch issues early.

1// In package.json, add to the "scripts" section:
2{
3  "scripts": {
4    "lint": "eslint src/",
5    "lint:fix": "eslint src/ --fix",
6    "build": "npm run lint && vite build"
7  }
8}