- Terraform – count, for_each and Dynamic Blocks
Three subnets should not be three copy-pasted blocks. `count` and its index, `for_each` over a map or a set, and the specific reason `count` will destroy and recreate half your resources when you remove one from the middle of a list. Then `for` expressions, `dynamic` blocks for repeated nested blocks, and the conditional expression.
- React Native – Taking Payments with Stripe
A worked native-module example that happens to involve money. Stripe's PaymentSheet rather than a card form of your own — the card never touches your code, which is what keeps you out of PCI scope. The two-step flow and why the order must exist before the sheet opens, treating a dismissed sheet as cancelled rather than failed, and the rule that outranks all of it: the device is never the authority on whether a payment succeeded.
- TypeScript – Narrowing and Type Guards
Narrowing is what makes unions usable: typeof, instanceof, the in operator, truthiness and equality all teach the checker something. Then the two you write yourself — a predicate returning `x is Thing`, and the discriminated union — plus the never trick that turns a forgotten case into a build failure instead of a blank badge.
- NestJS – Custom Decorators
The two kinds worth writing. createParamDecorator for pulling the current user out of a request, and SetMetadata plus Reflector for attaching data to a route that a guard reads back — the indirection that lets one guard serve every controller instead of each one writing its own check.
- Spring Study Guide – Get Started
Start here. What this guide is, who it is for, and the exact versions every answer is written against — Spring Framework 7 and Spring Boot 4.1 on Java 21. The eight topic areas in reading order, the pizza ordering API every snippet is lifted from, and the short list of things that changed between Spring 5 and Spring 7 that will make an old answer wrong.