typescript-expert

name	typescript-expert
description	TypeScript gotchas and decision criteria covering nullish coalescing pitfalls (|| vs ??), strict tsconfig settings (noUncheckedIndexedAccess, exactOptionalPropertyTypes), type guard patterns, discriminated unions, and as const vs enum. Use when writing TypeScript, configuring tsconfig, implementing type guards, or debugging null/undefined errors.
autoInvoke	false
priority	high
triggers	["typescript","type error","tsconfig","strict mode"]
paths	["**/*.ts","**/*.tsx","tsconfig.json","tsconfig.*.json"]
allowed-tools	Read, Grep, Glob, Edit, Write
user-invocable	false

AI-consumed reference. Optimized for Claude to read during execution. Human-readable explanation: see docs/architecture/HIERARCHICAL_PLANNING.md or docs/getting-started/ depending on topic.

TypeScript Expert — Gotchas & Decisions

Use Context7 for TypeScript docs.

Strict Config (Always Enable)

{ "strict": true, "noUncheckedIndexedAccess": true, "exactOptionalPropertyTypes": true }

Nullish Patterns

nullish[5]{bad,good,why}:
  "if (x)",if (x != null),"Empty string/0 are falsy but valid"
  "x || default","x ?? default","?? only catches null/undefined not falsy"
  "x!.prop",Type narrowing or optional chain,"! asserts non-null — hides bugs"
  "as Type",Type guard function,"as bypasses type checker"
  "any","unknown + narrowing","any disables ALL checking"

Type Guards

// User-defined type guard
function isUser(x: unknown): x is User {
  return typeof x === 'object' && x !== null && 'id' in x;
}

// Discriminated union (preferred for variants)
type Result<T> = { ok: true; data: T } | { ok: ; :  };