diff-cover-ts

A TypeScript reimplementation of diff_cover for the Node.js ecosystem.

Why this project?

The original diff_cover by Matt Bachmann is an excellent Python tool that enforces test coverage and code quality on git diffs. However, JavaScript/TypeScript projects that want to use it must install Python as a dependency in their CI pipelines.

diff-cover-ts provides the same functionality as a native Node.js package — no Python required. It can be run via npx, bunx, or yarn dlx, making it a drop-in fit for JS/TS toolchains.

Getting Started

Run in 30 seconds — no install required:

# 1. Generate a coverage report with your test runner
npx jest --coverage --coverageReporters=lcov      # Jest
npx vitest run --coverage                          # Vitest

# 2. Run diff-cover against the report
npx diff-cover-ts coverage/lcov.info

# 3. (Optional) Fail CI if coverage drops below 80%
npx diff-cover-ts coverage/lcov.info --fail-under 80

For code quality:

npx diff-cover-ts diff-quality --violations eslint --fail-under 90

See GETTING_STARTED.md for a full step-by-step tutorial, including CI setup.

Installation

# Run directly (no install needed)
npx diff-cover-ts <coverage-file>
bunx diff-cover-ts <coverage-file>

# Or install globally / as a dev dependency
bun add -d diff-cover-ts
npm install -D diff-cover-ts

Commands

diff-cover — Coverage on changed lines

Finds lines changed in a git diff that lack test coverage.

# Basic usage
diff-cover coverage/lcov.info

# Fail if less than 80% of changed lines are covered
diff-cover coverage/lcov.info --fail-under 80

# Multiple output formats at once
diff-cover coverage/cobertura.xml --format html:report.html --format json:coverage.json --format markdown:coverage.md

# Compare against a specific branch instead of origin/main
diff-cover coverage/lcov.info --compare-branch origin/develop

# Monorepo: restrict to one package's source files
diff-cover coverage/lcov.info --src-roots packages/core/src

# Only check certain files (glob patterns)
diff-cover coverage/lcov.info --include "src/**/*.ts" --exclude "src/**/*.test.ts"

# Use two-dot diff notation (uncommitted changes only)
diff-cover coverage/lcov.info --diff-range-notation ..

# Load settings from a TOML config file
diff-cover coverage/lcov.info --config-file pyproject.toml

diff-quality — Quality violations on changed lines

Finds code quality violations on lines changed in a git diff.

# Run ESLint on changed files and fail below 90% clean
diff-quality --violations eslint --fail-under 90

# Run Stylelint on changed CSS/SCSS files
diff-quality --violations stylelint

# Use a pre-generated lint report (any tool that outputs file:line:message)
diff-quality --violations generic --input lint-output.txt

# Compare against a specific branch and output Markdown
diff-quality --violations eslint --compare-branch origin/develop --format markdown:quality.md

# Suppress console output (useful in CI when you only want the file)
diff-quality --violations eslint --format json:quality.json --quiet

Supported Formats

Coverage reports

Format	File type
LCOV	.info / .lcov
Cobertura	XML
Clover	XML
JaCoCo	XML

Quality tools

Tool	How it runs
ESLint	npx eslint --format=compact
Stylelint	npx stylelint --formatter=compact
Generic	Reads file:line:message from --input files

Output formats

Console (default), HTML, JSON, Markdown — can output multiple formats at once.

CLI Options

Option	Description	Default
--compare-branch <branch>	Branch to compare against	origin/main
--fail-under <number>	Exit code 1 if coverage/quality % is below threshold	0
--include <glob...>	Only include matching files
--exclude <glob...>	Exclude matching files
--format <type[:path]>	Output format (console, html:path, json:path, markdown:path)	console
--diff-range-notation <n>	Git diff range notation (... or ..)	...
--src-roots <paths>	Comma-separated source roots (for monorepos)
--config-file <path>	TOML configuration file
--quiet	Suppress non-error output	false

Configuration File

Options can be set in a TOML file:

[tool.diff_cover]
compare_branch = "origin/develop"
fail_under = 80

[tool.diff_quality]
violations = "eslint"
fail_under = 90

diff-cover coverage.xml --config-file pyproject.toml

Programmatic API

import { runDiffCover, runDiffQuality } from 'diff-cover-ts'

const { report, exitCode } = await runDiffCover({
  coverageFiles: ['coverage/lcov.info'],
  compareBranch: 'origin/main',
  failUnder: 80,
  include: [],
  exclude: [],
  formats: [],
  diffRangeNotation: '...',
  quiet: false,
  srcRoots: [],
})

console.log(`Coverage: ${report.percentCovered}%`)

import { runDiffQuality } from 'diff-cover-ts'

const { report, exitCode } = await runDiffQuality({
  violations: 'eslint',
  inputFiles: [], // empty = run eslint directly
  compareBranch: 'origin/main',
  failUnder: 90,
  include: ['src/**/*.ts'],
  exclude: [],
  formats: [{ type: 'json', path: 'quality.json' }],
  diffRangeNotation: '...',
  quiet: false,
  srcRoots: [],
})

for (const file of report.files) {
  if (file.violations.length > 0) {
    console.log(`${file.path}: ${file.violations.length} violation(s)`)
  }
}
process.exit(exitCode)

Troubleshooting

diff-cover reports 0 changed lines

• Make sure you have uncommitted changes or commits that differ from --compare-branch.
• Try git diff origin/main...HEAD to verify the diff manually.
• Check that the coverage file paths match the git-relative paths in the diff.

Coverage file not found / not parsed

• LCOV files must end in .info or .lcov, or be passed by explicit path.
• XML files are auto-detected (Cobertura, Clover, JaCoCo). Verify with head -5 <file>.
• For monorepos, use --src-roots to strip path prefixes in coverage reports.

ESLint / Stylelint not found

• The quality drivers run npx eslint / npx stylelint and require those packages in your project.
• Alternatively, pre-generate a report and pass it via --input.

--fail-under is always 0

• If no changed lines are measurable (no coverage data matched), the percent is null and the threshold is not applied — this is intentional.

Credits

This project is inspired by and based on the design of diff_cover by Matt Bachmann. All credit for the original concept goes to that project.

License

MIT