How to Lint Only Changed Files in CI

Passing only the changed files to ESLint keeps PR lint time proportional to the diff, though it can miss violations that pre-existed in untouched files.

Compute the changed file list from the diff, filter to lintable extensions, and pass them to ESLint. This is ideal for fast PR feedback; keep a full lint on main so latent issues still surface.

Steps

• Get changed files with tj-actions/changed-files or git diff.
• Filter to lintable extensions with a files: glob.
• Pass the list to ESLint only when there is at least one file.

Workflow

steps:
  - uses: actions/checkout@v4
    with: { fetch-depth: 0 }
  - uses: actions/setup-node@v4
    with: { node-version: 20 }
  - run: npm ci
  - id: changed
    uses: tj-actions/changed-files@v45
    with:
      files: '**/*.{js,ts,tsx}'
  - if: steps.changed.outputs.any_changed == 'true'
    run: npx eslint ${{ steps.changed.outputs.all_changed_files }}

Gotchas

• Linting only the diff will not catch a rule violation that already lives in a file you did not touch; run a full lint on the default branch.
• A rule change (config edit) should trigger a full lint, since it can flag files that were previously clean.

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