TWIL August 7th 2026

• Even if a helper function isn't an observer itself, if it's used in an observer it re-runs in response to changes
  • So you need to convert it to a hook with useState to ensure a stable value if that's what you want
• You also can't just set it as the initial value of a useState or useRef if you don't want the logic to run
  • Since those still run on each render despite being discarded
• State in a hook is not kept when the component rendering that hook unmounts (duh)
  • But if you set it in your Mobx store on initialization it will
  • Took way too long for me to arrive at that solution
• There's something deeply satisfying about writing e2e tests, especially with Playwright's UI mode
  • admittedly my console.log-lead approach to debugging sometimes runs into issues with objects not printing like they would in Firefox
  • and it's incredibly annoying that I can't even copy text from the snapshots
  • but overall I find the experience meditative
• Before trying to Navigate somewhere with React Router, check the origin component actually knows about the destination route
  • Apparently some codebases have separate entrypoints with access to only a few routes
  • Or even no routes despite being wrapped in a Router
  • Probably for code-splitting?
• In RSpec, rather than using allow_any_instance_of prefer something like this

mockClass = instance_double(klass)
allow(klass).to receive(:new).and_return(mockClass)
allow(mockClass).to receive(:method)

It does fall apart a bit if multiple methods are called on the mocked instance though, since they can't be passed through. Each has to be mocked individually. Promotes testability I guess but if you're working in a complex legacy codebase...

Links

• How the Heck Does JPEG Work? - Shri Khalpada

Another banger from this series, and this time somewhat related to my usual areas of interest!