eslint-plugin-crap (aka crap4ts)

CRAP (Change Risk Anti-Pattern) metric for TypeScript/JavaScript, as an oxlint JS plugin.

Combines cyclomatic complexity with test coverage to flag functions that are both complex and under-tested — the riskiest code to change. A port of crap4clj to the TS ecosystem.

How it works

1. You run your tests with an lcov coverage reporter (vitest, jest, etc.), producing coverage/lcov.info.
2. The crap/crap lint rule computes cyclomatic complexity per function, looks up the function's line coverage in the lcov file, and reports any function whose CRAP score exceeds the threshold.

CRAP(fn) = CC² × (1 - coverage)³ + CC

| Score | Risk | |-------|------| | 1-5 | Low — clean code | | 5-30 | Moderate — refactor or add tests | | 30+ | High — complex and under-tested |

Setup

Requires oxlint with JS plugin support (v1.78+). In .oxlintrc.json:

{
  "jsPlugins": ["eslint-plugin-crap"],
  "rules": {
    "crap/crap": ["warn", { "maxCrap": 30, "lcovPath": "coverage/lcov.info" }]
  }
}

(oxlint strips the eslint-plugin- prefix, so rules are referenced as crap/....)

Prefer the classic crap4ts name? Alias it:

{
  "jsPlugins": [{ "name": "crap4ts", "specifier": "eslint-plugin-crap" }],
  "rules": {
    "crap4ts/crap": ["warn", { "maxCrap": 30 }]
  }
}

Make sure your test runner emits lcov. For vitest:

// vitest.config.ts
export default defineConfig({
    test: { coverage: { provider: 'v8', reporter: ['text', 'lcov'] } },
});

Usage

npx vitest run --coverage   # 1. generate coverage/lcov.info
npx oxlint .                # 2. lint with CRAP scores

Example output:

src/rule.ts:33:10: warning crap(crap): 'functionName' has a CRAP score of 40.4
(complexity 13, coverage 45.5%) — max is 30. Add tests or simplify.

Options

| Option | Default | Meaning | |--------|---------|---------| | maxCrap | 30 | Report functions scoring above this | | lcovPath | coverage/lcov.info | lcov file, relative to the lint root | | warnMissing | true | Warn when the lcov file is missing or a file has no entry in it |

Notes

• If the lcov file is missing, or a file has no entry in it, the rule reports a warning at the top of each linted file so the gap isn't silent. Set warnMissing: false to restore the old silent behaviour (e.g. in CI stages that lint before running coverage).
• If a source file is newer than the lcov file, the rule warns that coverage may be stale but still reports CRAP scores from the existing data.
• Functions whose line range contains no instrumented lines are skipped.
• Complexity counts: if, ?:, for/for-in/for-of, while, do-while, non-default case, catch, &&/||/??, and &&=/||=/??=, plus 1. Nested functions count toward their enclosing function too.

Why not ast-grep?

ast-grep rules are declarative pattern matchers — they can't count decision points, do arithmetic, or read a coverage file, so they can't compute CRAP. oxlint JS plugins run real JavaScript, so they can do all three.

Development

pnpm build             # compile src/ to dist/ for oxlint consumers
pnpm test              # unit + RuleTester tests
pnpm coverage          # regenerate lcov
pnpm lint              # build, then dogfood the rule on this repo