We plant a bug, then watch your tests.

Every line changed this month, checked for a test that executes it AND asserts on what it does.

it("keeps the minus sign on zero-value reversals", () => {
  expect(exportCsv(reversal(0)).row).toContain("-0.00");
});

2,148 mutations planted across 214 files, operators weighted by the repository’s own bug history. What the suite misses is a bug it would ship.

it("charges the full refund, not the rounded one", () => {
  expect(refundTotal(0.01, { qty: 3 })).toBe(-0.03);
});

34 merged fixes re-broken overnight, one at a time. A regression a test catches is guarded; a regression nothing notices would ship silently today.

it("rounds half-cents away from the customer", () => {
  expect(round(0.005)).toBe(0.01);
});

it("posts once when the retry times out", () => {
  expect(postWithRetry(timedOut(entry)).count).toBe(1);
});

Bugs written the way engineers actually write them — a model plants the mistakes a tired reviewer approves, and the suite either objects or does not.

-  const refundable = capture.amount;+  const refundable = order.total;

it("refunds only what was captured", () => {
  expect(refund(capture(order, 0.6))).toBe(-0.6);
});

Five properties of the ledger that must survive any refactor, attacked overnight with more than 3,000 generated inputs. Four held.

refund(0.01, qty: 3)
expected  sum(ledger) = 0
got       −0.02

property(amounts(), quantities(), (a, q) => {
  expect(sum(ledger(a, q))).toBe(0);
});