diff-coverage

A CLI tool that measures Jest/Vitest test coverage only for files changed in a git diff.

Instead of measuring whole-project coverage, diff-coverage focuses on the lines you actually changed — giving you actionable, targeted feedback.

Features

• Measures coverage only for changed files (not the entire codebase)
• Supports both Jest and Vitest (auto-detected)
• Per-file coverage breakdown with uncovered line numbers
• Configurable pass/fail threshold (exits with code 1 on failure)
• JSON output for programmatic use
• GitHub PR review comments with coverage gaps (review command)

Installation

npm install -g diff-coverage

Or run without installing via npx:

npx diff-coverage measure --cwd /path/to/your/project

CLI Usage

Basic — compare against main

diff-coverage measure --cwd /path/to/your/project

All options

diff-coverage measure \
  --cwd /path/to/project \              # project root (default: cwd)
  --base main \                          # base branch to diff against (default: merge-base of HEAD and main)
  --runner jest \                        # test runner: jest | vitest | auto (default: auto)
  --cmd "npx jest" \                     # override test runner command
  --ext "ts,tsx" \                       # file extensions to include (default: ts,tsx,js,jsx)
  --threshold 80 \                       # fail if line coverage is below this % (exit code 1)
  --exclude "*.mocks.ts,src/fixtures/**" # glob patterns to exclude
  --json                                 # output results as JSON
  --diff-only                            # show changed files only, skip running tests

List changed files without running tests

diff-coverage measure --diff-only --cwd /path/to/project

Example output

=== Diff Coverage Report ===

Files changed: 3
Lines:      72.5% (58/80)
Statements: 70.0% (56/80)
Functions:  66.7% (4/6)
Branches:   50.0% (6/12)

Threshold: 80% → ❌ FAIL

--- Per File ---
✅ src/services/bookService.ts
   Lines: 90%  Stmts: 88%  Fns: 100%  Branches: 75%
⚠️ src/resolvers/bookResolver.ts
   Lines: 60%  Stmts: 58%  Fns: 50%  Branches: 40%
   Uncovered lines: 45, 67, 89, 102
❌ src/utils/parser.ts
   Lines: 30%  Stmts: 28%  Fns: 0%  Branches: 0%
   Uncovered lines: 12, 15, 18, 23, 34 ... (+8)

CI Usage

# .github/workflows/coverage.yml
- name: Check diff coverage
  run: |
    npx diff-coverage measure \
      --cwd . \
      --base ${{ github.base_ref }} \
      --threshold 80

Jest / Vitest Configuration

diff-coverage works with your existing config with no changes required. For Jest, make sure coverageDirectory points to coverage/ (the default):

// jest.config.js
module.exports = {
  coverageDirectory: "coverage", // default — usually no change needed
}

Contributing

git clone https://github.com/ksugawara61/diff-coverage
cd diff-coverage
pnpm install
pnpm build
pnpm test

License

MIT