Our delivery analytics page loaded every GPS point for a city into the browser to draw a heatmap. At 1.2 million points the tab froze for 40 seconds and sometimes crashed. How we measured where the time went, moved aggregation to the server with grid binning, and rendered the result with a WebGL layer.
Invoices generated in the browser with jsPDF printed Polish, Vietnamese and Japanese names as random symbols. The 14 standard PDF fonts only cover a Latin-1-style character set. How we embedded a Unicode TTF font, subset it to keep files small, and tested names from every market.
Our session-end events showed up for about a third of sessions on desktop and almost none on mobile. The tracker posted with fetch inside an unload handler that browsers cut short or never fire. Flushing on visibilitychange with sendBeacon brought delivery above 97 percent.
Users editing several rows quickly lost some of their changes with no error at all. Our autosave saga used takeLatest, which cancelled the save for row A as soon as row B fired, leaving the UI saying Saved while state rolled back. Per-entity concurrency fixed it.
A refactor passed a class method straight to .then() and our sync client started crashing with Cannot read properties of undefined (reading 'retries'). Methods do not carry their object with them. We compare bind, arrow fields and wrappers, including the arrow-field trap with subclasses.
Video and dataset uploads of 5 to 15 GB kept failing through our API servers. We moved to browser-direct S3 multipart uploads with per-part presigned URLs, resumable state, per-part checksums and server-side completion. This is the full design and working code.
Our reading progress bar stopped at 92% and saved scroll positions were always slightly behind. A hand-written leading-edge throttle dropped the final scroll event of every gesture. We added a trailing call, tested it with fake timers, and compared requestAnimationFrame and scrollend.
Receipt uploads to S3 failed with 403 SignatureDoesNotMatch for about 4 percent of users, mostly iPhone photos uploaded from Windows. The URL was signed with one Content-Type and the browser sent another. Reading the CanonicalRequest in the error pinpointed it.
Our PDF invoices and the amounts charged through Stripe disagreed by a cent on 214 invoices in one month. Floats, toFixed and rounding at every step were to blame. We moved to integer minor units with one explicit rounding point.
A grid trading bot written in TypeScript sent quantities like 0.30000000000000004 and got error -1111. Binary floating point cannot represent most decimal fractions, and serialising numbers to strings exposed it. A decimal library end to end and string serialisation fixed it.
A batching test in our Node.js telemetry library failed about one run in twenty on CI and never locally. The code assumed setTimeout 0 fires before setImmediate, which Node does not guarantee in the main module. Moving to ESM later flipped nextTick and promise order too.
Our pricing page listed plans as 100, 1000, 25, 250. Array.prototype.sort() without a comparator converts values to strings and compares UTF-16 code units. We fixed every call site, handled nulls and mixed strings, and added a lint rule so it cannot return.
Event countdowns showed NaN days for iPhone users while Chrome was fine. Our API sent dates like 2024-12-3 9:05:00, which only some engines accept. We switched to strict ISO 8601 with an offset and stopped trusting Date parsing.
Our session timeout banner counted from 0 to 1 and then froze. The interval callback captured the first render's state, so it kept computing 0 + 1 forever. Functional state updates, and a ref-based useInterval hook for callbacks that read props, fixed it for good.