Linting Basics

8 examples to get you started with Linting & Formatting - 6 basic and 2 intermediate.

Prerequisites

• Node.js 24.18.0 project with TypeScript 5.6+.
• Install ESLint 9 flat config stack:

npm install -D eslint@9 typescript-eslint @eslint/js

Basic Examples

1. Minimal ESLint Flat Config

ESLint 9 uses eslint.config.js (or .mjs) instead of legacy .eslintrc.

// eslint.config.js
import eslint from "@eslint/js";
import tseslint from "typescript-eslint";
 
export default tseslint.config(
  eslint.configs.recommended,
  ...tseslint.configs.recommended,
  {
    ignores: ["dist/**", "node_modules/**"],
  }
);

• Flat config is an array of config objects merged in order.
• ignores replaces .eslintignore.
• typescript-eslint bundles parser and plugin for TS.

2. Add Lint Script

Wire lint into daily workflow and CI.

{
  "scripts": {
    "lint": "eslint .",
    "lint:fix": "eslint . --fix"
  }
}

• Run npm run lint before push; fail CI on errors.
• --fix handles import order and simple style rules automatically.

Related: Linting Best Practices - zero-warning policy

3. Type-Aware Rules

Enable type-checked linting for stricter correctness.

export default tseslint.config(
  ...tseslint.configs.recommendedTypeChecked,
  {
    languageOptions: {
      parserOptions: {
        projectService: true,
        tsconfigRootDir: import.meta.dirname,
      },
    },
  }
);

• projectService (TS 5.6+) simplifies parserOptions.project setup.
• Type-aware rules catch floating promises and unsafe any usage.
• Slightly slower; scope to src/** if needed.

4. Node Globals and ESM

Tell ESLint you run on Node with ES modules.

{
  files: ["**/*.ts"],
  languageOptions: {
    ecmaVersion: 2022,
    sourceType: "module",
    globals: {
      ...globals.node,
    },
  },
}

npm install -D globals

• sourceType: "module" matches "type": "module" in package.json.
• Prevents false no-undef on process and Buffer.

5. Ban console.log in Production Code

Keep logs structured via your logger, not raw console in src/.

{
  files: ["src/**/*.ts"],
  rules: {
    "no-console": ["error", { allow: ["warn", "error"] }],
  },
}

• Tests and scripts can use a separate config block with no-console: off.
• Pair with Pino or org logger in application code.

Related: Prettier Integration - formatting separate from lint

6. Lint Test Files Separately

Relax rules where mocks and any are common.

{
  files: ["test/**/*.ts", "**/*.test.ts"],
  rules: {
    "@typescript-eslint/no-explicit-any": "off",
    "no-console": "off",
  },
}

• Tests benefit from pragmatism; production src/ stays strict.
• Still ban unused vars in tests to catch dead setup code.

Intermediate Examples

7. Monorepo Root Config

Share one ESLint config across workspaces.

// eslint.config.js (root)
import base from "./packages/eslint-config/index.js";
 
export default [
  ...base,
  {
    files: ["apps/api/**/*.ts"],
    rules: { "no-console": "error" },
  },
];

• Extract shared config to packages/eslint-config for reuse.
• Apps extend with service-specific overrides only.

Related: Import Boundaries - architectural rules

8. CI Lint Gate

Fail PRs on lint errors and warnings if you adopt zero-warning policy.

- run: npm ci
- run: npm run lint -- --max-warnings 0

• --max-warnings 0 turns warnings into failures.
• Run lint after install, parallel to typecheck for speed.

Related: Typecheck in CI - complementary gate

Stack versions: This page was written for Node.js 24.18.0 (Active LTS), npm 10+, TypeScript 5.6+, Express 5, Fastify 5, and NestJS 11.