---
name: typescript-best-practices
description: TypeScript best practices. Use when reading or editing any .ts or .tsx file.
metadata:
  category: user-invoked
  version: 1.0.0
  tags: cursor-import, pstack
disable-model-invocation: true
---

## Pi Context-Aware Execution

When this skill is invoked in Pi, treat the user's current request and any skill arguments as the task input. Do not treat this file as the task by itself.

Before applying the skill, establish only the context needed for the request:

1. Identify the current working directory and relevant project scope.
2. Read local guidance first when present: AGENTS.md, CLAUDE.md, TODO.md, or nearby task/spec files.
3. Inspect the current codebase with targeted searches (prefer rg) and read relevant files before making claims or proposing changes.
4. Ground findings and recommendations in project evidence: cite file paths, commands, tests, docs, or external sources as applicable.
5. Ask a concise clarification only when the arguments and codebase context are insufficient to proceed safely.

Operate conservatively: avoid broad scans, large reads, subagents, or parallel fanout unless the user's requested depth clearly requires them.
Default output: return only the result, blockers, and required evidence. Omit preambles, process narration, repeated context, confidence scores, and follow-up offers. Use at most five bullets unless a required artifact or schema needs more.
# TypeScript best practices

Apply the **type-system-discipline** principle skill first; this skill grounds it in TypeScript syntax.

| 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 creation. |
| `unknown` over `any` | External data is `unknown`. `any` disables type checking everywhere it touches. |
| 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 | Validate where data crosses in; 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). |

Examples: `references/patterns.md`.