The CRAP Metric: Complexity and Coverage in One Number

Any coverage report plus any complexity tool gives you everything the formula needs.

// crap.js — coverage is a fraction, 0..1
export function crap(complexity, coverage) {
	const untested = 1 - coverage;
	return complexity ** 2 * untested ** 3 + complexity;
}

crap(15, 0);    // 240
crap(15, 0.5);  // 43.125
crap(15, 1);    // 15

A ratchet on changed methods keeps new risk out without opening a cleanup project nobody funded. Legacy scores stay visible on a dashboard instead of blocking every build.

# CI: score only the methods this branch touched
git diff --name-only origin/main... -- '*.js' \
  | xargs node ./tools/crap-report.mjs --threshold 30 --changed-only

# exit non-zero when a touched method crosses the line;
# print the untouched offenders as a report, not a failure

When the score comes from complexity rather than coverage, testing harder is the wrong response. Pull each branch out into something with complexity 1 and the driver stays flat no matter how many rules arrive.

// Before: complexity climbs with every rule anyone adds.
function validate(order) {
	if (!order.id) return 'missing id';
	if (order.items.length === 0) return 'no items';
	if (order.total < 0) return 'negative total';
	if (order.currency !== 'USD' && order.currency !== 'EUR') return 'bad currency';
	if (order.customer && !order.customer.email) return 'customer without email';
	return null;
}

// After: each rule is trivially testable, the driver stays at 2.
const RULES = [
	[(o) => !o.id, 'missing id'],
	[(o) => o.items.length === 0, 'no items'],
	[(o) => o.total < 0, 'negative total'],
	[(o) => !['USD', 'EUR'].includes(o.currency), 'bad currency'],
	[(o) => Boolean(o.customer) && !o.customer.email, 'customer without email']
];

function validate(order) {
	for (const [fails, message] of RULES) if (fails(order)) return message;
	return null;
}