We stopped adding patterns because a book listed them and kept only the three that removed real pain from a Node.js billing service: strategy for pricing rules, adapter for payment providers and plain constructor injection. Here is the before and after code.
After we turned on streaming for our assistant, tool calls failed with JSON parse errors and sometimes ran with another call's arguments. We were parsing argument fragments per chunk and keying them by arrival order. Accumulating by index 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.
A routine dependency update moved us to ESLint 9, and lint failed in CI and every editor: ESLint couldn't find an eslint.config file. ESLint 9 uses flat config by default and ignores .eslintrc. How we migrated a TypeScript and React project, kept Prettier out of the way, and avoided the common traps.
One CI run in eight failed somewhere in our checkout E2E suite, never in the same test twice. The causes were fixed sleeps, URL checks that raced navigation and text selectors that matched two buttons. Web-first assertions, role locators and traces fixed 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 billing test passed for weeks and failed this morning, 31 January, for every engineer at once. JavaScript's setMonth rolled 31 January into 3 March, and the test read the real clock. We fixed the arithmetic, injected the clock and added property tests over every day.
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.
After upgrading to Next.js 15 our dev console filled with sync dynamic API warnings, and one un-awaited auth helper quietly stopped redirecting logged-out users. How we migrated cookies(), headers() and params safely.