fastlint

1.1.0 verified Fri May 01 auth: no javascript

fastlint is a command-line tool (v1.1.0) that speeds up linting by running linters only on files changed in git — either in the working copy, staged, or compared to branches like master. It avoids running on unchanged files, integrating with eslint and other tools via piped filenames. Key features: --staged filter, --diff-filter (A/C/D/M/R/etc.), --glob filtering, and multiple path output modes. Stable, low maintenance, single-purpose tool. Alternatives include lint-staged and husky, but fastlint is more flexible for branch comparison.

Common errors

error fastlint: 'origin/master' is not a valid git reference ↓

cause Typo or remote not fetched; branch name incorrect.

fix

Use 'origin/main' or 'master' if local branch exists.

error fatal: ambiguous argument 'HEAD~5 HEAD': unknown revision or path ↓

cause Missing space between revisions and paths, or incorrect syntax.

fix

Use correct form: fastlint <since> <until>, e.g. 'HEAD~5 HEAD'.

error fastlint: option '--print0' requires a value ↓

cause Using --print0 as a boolean flag; it's fine, but if followed by argument, fails.

fix

Place --print0 at end of options before xargs.

Warnings

gotcha --diff-filter=Q includes untracked files, not 'Deleted' as some might assume. ↓

fix Use correct letter: Q for untracked, D for deleted.

gotcha Default behavior includes working copy changes if no branch arguments given; may surprise users expecting empty output. ↓

fix Always specify two branches or use --working-copy explicitly.

Install

npm install fastlint

yarn add fastlint

pnpm add fastlint

Imports

fastlint
wrong
```
require('fastlint')
```
correct
```
npx fastlint ...
```
fastlint is a CLI tool, not a library. Use via npx or global install.
fastlint binary
wrong
```
fastlint.start()
```
correct
```
fastlint --version
```
Always invoked from shell, not programmatically.

Quickstart

Installs fastlint, creates a git repo, makes a change, then lints only changed files with eslint.

npm install -g fastlint && echo "console.log('hello')" > test.js && git init && git add test.js && git commit -m "init" && echo "console.log('world')" >> test.js && fastlint HEAD~1 HEAD --print0 | xargs -0 npx eslint