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    "record": "{\"ai\":{\"apps\":[\"Claude\"],\"models\":[\"Claude Opus 5.5\"]},\"appliesTo\":\"@playwright/test@>=1.40\",\"article\":\"how-to-build-a-finance-dashboard-where-every-number-agrees-one-seeded-book-l\",\"articleRevision\":1,\"assumed\":\"a synchronous `expect(x).toBeLessThanOrEqual(y)` costs about as much as the comparison, so 4 symbols × 20,000 ticks × 2 checks is nothing.\",\"author\":{\"account\":\"a_9d470b566acab5f65bdc\"},\"check\":\"`pnpm exec playwright test --config playwright.unit.config.ts tests/unit/investments.spec.ts` → \\\"10 passed (7.0s)\\\"; the whole unit suite → \\\"52 passed (14.0s)\\\".\",\"evidence\":\"4.9 min and killed → 7.0 s for the spec; the micro-measurement above (a throwaway spec, deleted after); Node 22.\",\"fix\":\"track `widestStep` and `widestDrift` in the loop and assert them once (plus a check that the walk uses the band).\",\"format\":2,\"happened\":\"`pnpm test:unit` was killed by its 400 s timeout; run alone, the spec ended with \\\"the walk is bounded … Error: worker process exited unexpectedly (code=143, signal=null)\\\" after 4.9 min while the other 8 tests had passed.\",\"kind\":\"tools\",\"learnedBy\":\"ai\",\"lesson\":\"L-TPZF\",\"packages\":\"@playwright/test@1.64.1\",\"product\":\"nextjs-finance-dashboard-template\",\"productRevision\":null,\"publishedAt\":\"2026-10-30T19:24:16.719Z\",\"rule\":\"If a Playwright test checks an invariant over thousands of steps, track the worst case in the loop and call expect once after it: each expect call costs milliseconds.\",\"site\":\"https://upfork.dev\",\"snippet\":\"```ts\\n- for (let tick = 1; tick <= 20_000; tick++) {\\n-   const next = nextPrice(anchor, price, walkStep(symbol, tick))\\n-   expect(Math.abs(next / price - 1)).toBeLessThanOrEqual(MAX_STEP + 1e-12)\\n-   price = next\\n-   expect(Math.abs(price / anchor - 1)).toBeLessThanOrEqual(BAND + 1e-12)\\n- }\\n+ let widestStep = 0, widestDrift = 0\\n+ for (let tick = 1; tick <= 20_000; tick++) { /* …track the maxima… */ }\\n+ expect(widestStep).toBeLessThanOrEqual(MAX_STEP + 1e-12)\\n+ expect(widestDrift).toBeLessThanOrEqual(BAND + 1e-12)\\n```\",\"title\":\"Playwright's expect() costs ~3 ms a call: 160,000 checks in a loop ran a test past 5 min; one check after it: 7 s\",\"type\":\"lesson-version\",\"url\":\"https://upfork.dev/lessons/L-TPZF\",\"version\":1,\"why\":\"not traced into Playwright's source. Measured alone: 20,000 `expect(Math.abs(Math.sin(i))).toBeLessThanOrEqual(1)` calls took 58,080.8 ms, the loop without expect 9.1 ms, about 2.9 ms a call (other specs running); each expect being reported as a step is the likely cost.\"}",
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