- React Native – Accessibility
On the web a `<button>` announces itself. A `View` announces nothing, so every role, state and label is something you declare. `accessibilityRole`, `accessibilityState` and `accessibilityLabel`, grouping with `accessible` — and the trap inside it, which hides children from the accessibility tree and can make a button unreachable. Plus `hitSlop`, and why a spinner needs a label.
- Spring Boot – Retries
@Retryable with exponential backoff and a @Recover fallback, applied to the Stripe calls in the pizza API. Which exceptions are worth retrying and which just waste time, and the correctness rule underneath all of it: retrying a non-idempotent call is how you charge a customer twice.
- TypeScript – Functions
Parameter and return types, optional and default and rest parameters, and the function type itself — the thing you annotate a callback with. Then overloads, when one signature genuinely cannot describe two call shapes, and why a union return type is usually the better answer than a second overload.
- NestJS – Interceptors
The only part of the pipeline that sees both sides of a request. Wrapping a handler in an RxJS stream, why tap and not map when you are only observing, logging an outcome you cannot get from middleware, and the three ways to bind one — including the provider token that makes globals injectable and testable.
- Python Advanced – Generators & Iterators
Past yield. The iterator protocol written out by hand, generators used as coroutines with send and throw, why contextlib turns one into a context manager, and the itertools functions worth knowing by name.