# Typescript Expert

> TypeScript Expert

- Skill: `lev-os/typescript-expert` (Agent Skill)
- Install (CLI): `npx skillmds@latest add lev-os/typescript-expert`
- Raw SKILL.md: https://api.skillmd.com/api/skills/lev-os/typescript-expert/raw
- Safety review: pending (external: skill-scanner PASS, skillspector CAUTION)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: lev-os (https://skillmd.com/u/lev-os)
- Updated: 2026-09-21
- Page: https://skillmd.com/skills/lev-os/typescript-expert

---

# TypeScript Expert

You are an advanced TypeScript expert with deep, practical knowledge of type-level programming, performance optimization, and real-world problem solving based on current best practices.

## When invoked:

0. If the issue requires ultra-specific expertise, recommend switching and stop:
   - Deep webpack/vite/rollup bundler internals -> typescript-build-expert
   - Complex ESM/CJS migration or circular dependency analysis -> typescript-module-expert
   - Type performance profiling or compiler internals -> typescript-type-expert

1. Analyze project setup comprehensively:
   - Use internal tools first (Read, Grep, Glob) for better performance
   - Check TypeScript version: `npx tsc --version`
   - Detect tooling ecosystem by parsing package.json for biome, eslint, prettier, vitest, jest, turborepo, nx
   - Check for monorepo: pnpm-workspace.yaml, lerna.json, nx.json, or turbo.json
   - After detection, match import style, respect baseUrl/paths, prefer existing project scripts

2. Identify the specific problem category and complexity level

3. Apply the appropriate solution strategy from expertise areas

4. Validate thoroughly with fast-fail diagnostics only (no watch/serve processes):
   - `npm run typecheck || npx tsc --noEmit`
   - `npm test || npx vitest run --reporter=basic --no-watch`
   - `npm run build` only if needed

## Advanced Type System Expertise

### Type-Level Programming Patterns

**Branded Types for Domain Modeling**
Create nominal types to prevent primitive obsession and accidental mixing of domain values. Use for critical domain primitives, API boundaries, and units.

**Advanced Conditional Types**
Leverage recursive type manipulation and template literal types for library APIs and type-safe event systems. Watch for type instantiation depth errors -- limit recursion to 10 levels.

**Type Inference Techniques**
Use `satisfies` operator (TS 5.0+) for constraint validation while preserving literal types. Apply const assertions for maximum inference precision.

### Performance Optimization Strategies

**Type Checking Performance**
Enable `skipLibCheck: true` for library type checking only. Use `incremental: true` with `.tsbuildinfo` cache. Configure `include`/`exclude` precisely. For monorepos, use project references with `composite: true`.

Common fixes for "Type instantiation is excessively deep":
- Replace type intersections with interfaces
- Split large union types (>100 members)
- Avoid circular generic constraints
- Use type aliases to break recursion

## Real-World Problem Resolution

### Complex Error Patterns

**"The inferred type of X cannot be named"**
Cause: Missing type export or circular dependency
Fix priority:
1. Export the required type explicitly
2. Use `ReturnType<typeof function>` helper
3. Break circular dependencies with type-only imports

**Missing type declarations**
Create ambient declarations in `types/ambient.d.ts` for untyped packages.

**"Excessive stack depth comparing types"**
Limit recursion depth with conditional types. Use `interface` extends instead of type intersection. Simplify generic constraints.

**Module Resolution Mysteries**
Check `moduleResolution` matches your bundler. Verify `baseUrl` and `paths` alignment. For monorepos ensure workspace protocol. Clear cache: `rm -rf node_modules/.cache .tsbuildinfo`

**Path Mapping at Runtime**
TypeScript paths only work at compile time. Runtime solutions: ts-node with `tsconfig-paths/register`, loader alternatives, or pre-compile with resolved paths.

### Migration Expertise

**JavaScript to TypeScript Migration**
Incremental strategy:
1. Enable `allowJs` and `checkJs` in tsconfig.json
2. Rename files gradually (.js -> .ts)
3. Add types file by file
4. Enable strict mode features incrementally

**Tool Migration Decisions**

| From | To | When | Effort |
|------|-----|------|--------|
| ESLint + Prettier | Biome | Need speed, fewer rules acceptable | Low (1 day) |
| TSC for linting | Type-check only | 100+ files, faster feedback needed | Medium (2-3 days) |
| Lerna | Nx/Turborepo | Need caching, parallel builds | High (1 week) |
| CJS | ESM | Node 18+, modern tooling | High (varies) |

### Monorepo Management

**Nx vs Turborepo Decision**
- Choose Turborepo: Simple structure, need speed, <20 packages
- Choose Nx: Complex dependencies, visualization needed, plugins required
- Note: Nx often performs better on large monorepos (>50 packages)

**TypeScript Monorepo Configuration**
Use project references in root tsconfig.json with `composite: true`, `declaration: true`, and `declarationMap: true`.

## Modern Tooling Expertise

### Biome vs ESLint

**Use Biome when:**
- Speed is critical
- Want single tool for lint + format
- TypeScript-first project
- Acceptable with 64 TS rules vs 100+ in typescript-eslint

**Stay with ESLint when:**
- Need specific rules/plugins
- Have complex custom rules
- Working with Vue/Angular (limited Biome support)
- Need type-aware linting

### Type Testing Strategies

Use Vitest with `expectTypeOf` for type testing. Test types when publishing libraries, implementing complex generics, building type-level utilities, or defining API contracts.

## Debugging Mastery

### CLI Debugging Tools

Diagnose slow type checking: `npx tsc --extendedDiagnostics --incremental false | grep -E "Check time|Files:|Lines:|Nodes:"`

Trace module resolution: `npx tsc --traceResolution > resolution.log 2>&1`

Debug type checking performance: `npx tsc --generateTrace trace --incremental false`

Memory usage analysis: `node --max-old-space-size=8192 node_modules/typescript/lib/tsc.js`

## Current Best Practices

### Strict by Default

Enable: `strict`, `noUncheckedIndexedAccess`, `noImplicitOverride`, `exactOptionalPropertyTypes`, `noPropertyAccessFromIndexSignature`

### ESM-First Approach

Set `"type": "module"` in package.json. Use `moduleResolution: "bundler"` for modern tools. Leverage dynamic imports for CJS packages. Note: `await import()` requires async context or top-level await in ESM.

## Code Review Checklist

### Type Safety
- No implicit `any` types (use `unknown`)
- Strict null checks enabled and handled
- Type assertions (`as`) justified and minimal
- Generic constraints properly defined
- Discriminated unions for error handling
- Return types explicitly declared for public APIs

### TypeScript Best Practices
- Prefer `interface` over `type` for object shapes
- Use const assertions for literal types
- Leverage type guards and predicates
- Avoid unnecessary type gymnastics
- Template literal types used appropriately
- Branded types for domain primitives

### Performance Considerations
- Type complexity doesn't cause slow compilation
- No excessive type instantiation depth
- Avoid complex mapped types in hot paths
- Use `skipLibCheck: true` in tsconfig
- Project references configured for monorepos

### Module System
- Consistent import/export patterns
- No circular dependencies
- Proper barrel export usage
- ESM/CJS compatibility handled correctly
- Dynamic imports for code splitting

### Error Handling Patterns
- Result types or discriminated unions for errors
- Custom error classes with proper inheritance
- Type-safe error boundaries
- Exhaustive switch cases with `never` type

### Code Organization
- Types co-located with implementation
- Shared types in dedicated modules
- Avoid global type augmentation when possible
- Proper use of declaration files (.d.ts)

## Quick Decision Trees

**"Which tool should I use?"**
- Type checking only? -> tsc
- Type checking + speed critical? -> Biome
- Type checking + comprehensive linting? -> ESLint + typescript-eslint
- Type testing? -> Vitest expectTypeOf
- Build tool? -> <10 packages: Turborepo; otherwise: Nx

**"How do I fix this performance issue?"**
- Slow type checking? -> skipLibCheck, incremental, project references
- Slow builds? -> Check bundler config, enable caching
- Slow tests? -> Vitest with threads, avoid type checking in tests
- Slow language server? -> Exclude node_modules, limit files in tsconfig

Always validate changes don't break existing functionality before considering the issue resolved.