The same customer shows up in a list, a detail page, and a header badge — three copies of one truth you don't own. Edit one and the others go stale. Here's when to normalize server data into one source of truth, when the query cache is enough, and how to invalidate without nuking everything.
localStorage looks like a global variable that survives a refresh. That framing is the bug. Treat it as a persistence layer that mirrors your state — not as the state itself — and the hydration mismatches, stale tabs, and schema breakages mostly disappear.
A shared UI kit does not fail because the button component is hard to write. It fails when five applications are afraid to upgrade it. Versioning, contract tests, release channels, and migration paths are what turn a component library from a dependency into a product people can trust.
Writing the new version of a shared component is the easy 10%. The other 90% is the hundred call sites that already depend on the old one — and the fear of touching them. Here's how I change a component everyone uses without a month-long rewrite that breaks the day it lands.
The most reusable UI code I write renders nothing at all. Headless components, render props, and hooks-as-API are three names for one idea: separate what a component does from how it looks — and hand over the behavior without a single opinion about the markup.
The moment a component grows past a handful of props, you're not writing a component anymore — you're designing an API. Compound components are how you keep that API flexible without drowning it in configuration props.
The pattern that taught a generation to split 'smart' from 'dumb' components got quietly declared obsolete when hooks arrived. It wasn't. The folder convention died. The idea underneath it is more useful now than ever — including in Server Components.
Split a component too early and you get a maze of tiny files. Split too late and you get a 600-line monster. The real question isn't size — it's who owns which dependency, and whether the component reaches for it or is handed it.
You can draw whatever folder diagram you like. The honest picture of your app is which module imports which — the direction of the arrows. Get that wrong and no folder structure will save you.
A barrel file makes imports look beautiful and can quietly drag half your app into a bundle that only needed one function. Here's what index.ts actually does, when it earns its place, and when it's a trap wearing a clean interface.