Linting in CI/CD

Run ESLint, Prettier, and TypeScript type-checking in GitHub Actions to enforce code quality on every pull request.

Recipe

Quick-reference recipe card - copy-paste ready.

// package.json scripts
{
  "scripts": {
    "lint": "next lint",
    "format:check": "prettier --check .",
    "type-check": "tsc --noEmit"
  }
}

# Run all checks locally (same as CI)
npm run lint && npm run format:check && npm run type-check

When to reach for this: Every project that uses pull requests. CI is your safety net for catching issues that pre-commit hooks miss.

Working Example

# .github/workflows/code-quality.yml
name: Code Quality
 
on:
  pull_request:
    branches: [main]
  push:
    branches: [main]
 
concurrency:
  group: ${{ github.workflow }}-${{ github.ref }}
  cancel-in-progress: true
 
jobs:
  quality:
    name: Lint, Format & Type Check
    runs-on: ubuntu-latest
    timeout-minutes: 10
 
    steps:
      - name: Checkout
        uses: actions/checkout@v4
 
      - name: Setup Node.js
        uses: actions/setup-node@v4
        with:
          node-version: 20
          cache: "npm"
 
      - name: Install dependencies
        run: npm ci
 
      - name: ESLint
        run: npm run lint
 
      - name: Prettier
        run: npm run format:check
 
      - name: TypeScript
        run: npm run type-check

What this demonstrates:

• Three checks run in sequence: ESLint, Prettier format check, and TypeScript type check
• npm ci installs exact versions from lock file (faster and deterministic)
• cache: "npm" caches node_modules between runs for speed
• concurrency cancels in-progress runs when new commits are pushed
• timeout-minutes prevents stuck jobs from running indefinitely

Deep Dive

How It Works

• GitHub Actions runs the workflow on every pull request and push to main
• Each step exits with a non-zero code on failure, which marks the PR check as failed
• npm run lint runs next lint, which exits with code 1 if there are errors
• prettier --check exits with code 1 if any file is not formatted correctly
• tsc --noEmit exits with code 1 if there are type errors
• Failed checks block merging when branch protection rules require them

Variations

Parallel jobs (faster for large projects):

jobs:
  lint:
    name: ESLint
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-version: 20
          cache: "npm"
      - run: npm ci
      - run: npm run lint
 
  format:
    name: Prettier
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-version: 20
          cache: "npm"
      - run: npm ci
      - run: npm run format:check
 
  type-check:
    name: TypeScript
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-version: 20
          cache: "npm"
      - run: npm ci
      - run: npm run type-check

With Biome (single check replaces lint + format):

- name: Biome
  run: npx biome check .

PR review comments with reviewdog:

- name: ESLint with reviewdog
  uses: reviewdog/action-eslint@v1
  with:
    github_token: ${{ secrets.GITHUB_TOKEN }}
    reporter: github-pr-review
    eslint_flags: "src/"

This posts ESLint errors as inline PR review comments on the exact lines that need fixing.

Caching for pnpm:

- name: Setup pnpm
  uses: pnpm/action-setup@v4
  with:
    version: 9
 
- name: Setup Node.js
  uses: actions/setup-node@v4
  with:
    node-version: 20
    cache: "pnpm"
 
- name: Install dependencies
  run: pnpm install --frozen-lockfile

Branch protection setup:

GitHub repo → Settings → Branches → Branch protection rules:
  ✓ Require status checks to pass before merging
  ✓ Require branches to be up to date before merging
  Status checks: "Lint, Format & Type Check" (or individual job names)

TypeScript Notes

// tsc --noEmit checks the ENTIRE project, not just changed files.
// This is intentional - a change in one file can break types elsewhere.
 
// Example: changing a shared type
// types.ts
export type User = {
  name: string;
  email: string;
  role: "admin" | "user"; // Adding "moderator" here is safe
};
 
// But removing "admin" breaks every file that uses User.role === "admin"
// Only tsc catches this - ESLint cannot.

Gotchas

Things that will bite you. Each gotcha includes what goes wrong, why it happens, and the fix.

• CI passes but local fails (or vice versa) - Different Node.js versions, different dependency versions, or OS-specific behavior. Fix: Pin Node.js version in CI to match local. Use npm ci (not npm install) to use exact lock file versions.

• Lint errors on generated files - CI lints files that are generated during the build (e.g., Prisma client, GraphQL types). Fix: Add generated directories to .eslintignore or the ignores array in eslint.config.mjs and .prettierignore.

• tsc is slow in CI - TypeScript type-checking can take 30 seconds or more on large projects. Fix: Enable "incremental": true in tsconfig.json and cache the .tsbuildinfo file between CI runs. Alternatively, use @vercel/next-swc which does type-checking faster.

• Prettier check fails on line endings - Windows developers commit files with CRLF, CI runs on Linux with LF. Fix: Set "endOfLine": "lf" in .prettierrc and configure Git: git config --global core.autocrlf input.

• Branch protection not enforced - Status checks only block merging if branch protection is configured. Without it, anyone can merge failing PRs. Fix: Enable branch protection rules on main and require the CI job to pass.

Alternatives

Other ways to solve the same problem - and when each is the better choice.

FAQs

concurrency:
  group: ${{ github.workflow }}-${{ github.ref }}
  cancel-in-progress: true

- name: Biome
  run: npx biome check .

Related

• ESLint Setup for Next.js - ESLint configuration that CI runs
• Prettier Setup - Prettier configuration for format checks
• TypeScript Strict Mode - tsconfig options that tsc --noEmit enforces
• Husky & lint-staged - pre-commit hooks complement CI
• ESLint + Prettier Integration - ensure consistent behavior between local and CI