Engineering learnings

TL;DR — Frontend/state/testing bug classes that keep recurring, and the fixes that stuck. Written so future-me stops re-learning them. (Demo content.)

State & lifecycle

• Never clear local data on sign-out. In PixelPaws the daily streak lived only in the local blob; the cloud copy stored a high score, not the streak. Wiping local state on logout deleted the streak for good. Sign-out should end the session and keep local data.
• Cancel before you start. A render loop started without cancelling the previous requestAnimationFrame runs twice as fast on replay. Cancel at the top of start().
• Guard double-submit synchronously. Set the "busy" flag before any await, not after — otherwise a fast second click slips through.

Caching & PWAs

• Bump the asset version when you edit a non-hashed file, or the service worker keeps serving the old one.
• Network-first for documents, cache-first for hashed assets. Mixing them up is why "my change didn't deploy" is usually a cache, not a build.

Testing

• Test the pure logic, not the DOM. If logic is hard to test, it's too coupled to the view — split it. See learnings/README.md.