/typescript-best-practices

name: typescript-best-practices
description: TypeScript best practices. Use when reading or editing any .ts or .tsx file.
paths: ["**/*.ts", "**/*.tsx"]
disable-model-invocation: true

TypeScript best practices

Apply the **type-system-discipline** principle skill first.

| Rule | Summary | |------|---------| | Discriminated unions | Model variants with a `kind` literal discriminant so impossible states can't be represented. No optional-field bags. | | Branded types | Brand primitives with `& { readonly __brand: "X" }` so they can't be mixed up. Validate once at the boundary. | | Constructive modeling | Build the shape so the illegal value can't be constructed. `[T, ...T[]]` for non-empty, `[T, T][]` for even length, `start` plus `duration` for a range. Not a runtime guard, not a wish for refinement types. | | Simplest total type | Keep `T[]` while every operation on it stays total. Strengthen to `NonEmpty<T>` only where the loose type forces `!`, a cast, or a "should never happen" throw. | | `unknown` over `any` | External data is `unknown`. | | Schemas before guards | Before hand-writing a property-by-property type guard, use the repository's runtime schema library and infer the type from the schema, such as `z.infer`. | | No `as` casts | Every `as` is a runtime crash waiting. Cast only after validation. | | Narrowing hierarchy | Discriminant switch > `in` operator > `typeof`/`instanceof` > user-defined type guard > `as`. | | Type guards | Must verify the claim. A lying guard is worse than `as` because the bug hides behind a name that says it's safe. Name them `isX` or `hasX`. | | Exhaustiveness | Inline `const _exhaustive: never = x;` in default arms so the compiler errors when a new variant is added. | | `satisfies` over `as` | Validates the value without widening literal types. | | Boundary validation | Parse where data crosses in, into a named domain type. `Record<string, unknown>` (however spelled) stops at that parse. Trust types inside. See the **boundary-discipline** principle skill. | | Schema-derived types | Reach for `Pick`/`Omit`/`Parameters`/`ReturnType`/`Awaited`/`typeof` before declaring a new interface. | | Object args | Pass objects, not positional, so argument order is self-documenting. Skip on hot paths (per-frame render, tokenizers, parsers). | | Real tests | Don't mock what you can run. Prefer the framework's real test primitives with leak/disposable checks, and verify UI in a running build. Mock only what you can't run locally. | | Structured telemetry | Prefer structured logger diagnostics with enough context to debug from an id. No `console.log` in shipped code. |

Examples: `references/patterns.md`.