---
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`.

## Session capabilities and permissions

Read [host capabilities](../../references/host-capabilities.md) for the current session. Read the [type-system-discipline](../principle-type-system-discipline/SKILL.md) principle first, and the [boundary-discipline](../principle-boundary-discipline/SKILL.md) principle for boundary validation. Use the complete [patterns reference](references/patterns.md). If a dependency cannot be read, name it and label the review partial.

Apply these practices to supplied code or readable repository files. Saved edits require available write tools and authorization for the affected paths. If writes are denied or unavailable, return a proposed refactor in the conversation and state that it is unsaved. If no TypeScript input can be read or supplied, report the missing input before proceeding.

Run the repository's checks only with available execution tools and authorized commands. Reuse its installed runtime schema library; do not install a dependency for one guard. Report compiler, test, and running UI checks only after observing their results. If execution is denied or unavailable, identify the checks that remain unperformed. Retain the source metadata without assuming that every host enforces paths or disable-model-invocation.