write-service-codelisted

## When to use Reach for this when you're writing the **body** of a handler/service/query/event/test and want the team's implementation conventions — readability, async, nullability, SQL performance, messaging, logging, and test shape. For *where files go and what they're named*, use the companion skill [structure-a-backend-service](./structure-a-backend-service.md). Each rule: **portable principle** → **▸ Example (TS/NestJS)** (neutral `listing` domain; `<Entity>`/`<feature>` = rename) → **▸ Other stacks**. Some rules restate global policy; the global rule stays authoritative (see Related). ## Steps ### 1. Control flow — return early, no manual loops - **Return (or throw) early; don't nest.** Handle the invalid/empty case first and bail, so the happy path stays unindented. ```ts if (minPrice == null && maxPrice == null) return; // guard if (!items?.length) return null; // ...happy path, never more than ~2 levels deep ``` - **Transform collections with pipeline functions, not `for`/`while`.** `map`/`filter`/`reduce`/ `find`/`some`/`every`/`flatMap`. Keep callbacks pure (no mutating the source). ```ts const coverIds = media.filter((m) => m.type === MediaType.COVER).map((m) => m.id); const labelByCode = categories.reduce((acc, c) => { acc[c.code] = c.label; return acc; }, {} as Record<string, string>); ``` - **Imperative iteration only for:** side effects (`forEach` / `for...of` to mutate external state, dispatch, log) and **sequential async tha