Imported from shipshitdev/skills (skills/typescript-expert/SKILL.md). Install upstream with npx skills add shipshitdev/skills --skill typescript-expert. Copyright stays with the author.

TypeScript Expert

When invoked

1. Analyze project setup comprehensively:

   Prefer built-in file-reading and search capabilities for performance. Shell commands are fallbacks.

   # Core versions and configuration
   bunx tsc --version
   node -v
   # Detect tooling ecosystem (prefer parsing package.json)
   node -e "const p=require('./package.json');console.log(Object.keys({...p.devDependencies,...p.dependencies}||{}).join('\n'))" 2>/dev/null | grep -E 'biome|eslint|prettier|vitest|jest|turborepo|nx' || echo "No tooling detected"
   # Check for monorepo (fixed precedence)
   (test -f pnpm-workspace.yaml || test -f lerna.json || test -f nx.json || test -f turbo.json) && echo "Monorepo detected"

   After detection, adapt approach:

   • Match import style (absolute vs relative)
   • Respect existing baseUrl/paths configuration
   • Prefer existing project scripts over raw tools
   • In monorepos, consider project references before broad tsconfig changes

2. Identify the specific problem category and complexity level

3. Apply the appropriate solution strategy from the expertise below

4. Validate thoroughly:

   # Fast fail approach (avoid long-lived processes)
   bun run typecheck || bunx tsc --noEmit
   bun run test || bunx vitest run --reporter=basic --no-watch
   # Only if needed and build affects outputs/config
   bun run build

   Safety note: Avoid watch/serve processes in validation. Use one-shot diagnostics only.

Advanced Type System Expertise

Type-Level Programming Patterns

Branded Types for Domain Modeling — nominal types (type UserId = Brand<string, 'UserId'>) prevent accidentally mixing domain primitives that share a base type. Use for critical domain primitives, API boundaries, currency/units. See references/typescript-cheatsheet.md (§ Branded Types) for the full pattern. Resource: https://egghead.io/blog/using-branded-types-in-typescript

Advanced Conditional Types — recursive type manipulation (e.g. DeepReadonly<T>) and template-literal event-source APIs. Use for library APIs, type-safe event systems, compile-time validation. Watch for type instantiation depth errors (limit recursion to 10 levels). See references/typescript-cheatsheet.md (§ Conditional Types, § Mapped Types, § Template Literal Types).

Type Inference Techniques — use satisfies (TS 5.0+) for constraint validation while preserving literal types; use as const assertions for maximum inference on literal arrays/objects. See references/typescript-cheatsheet.md (§ Best Practices).

Performance Optimization Strategies

Type Checking Performance

bunx tsc --extendedDiagnostics --incremental false | grep -E "Check time|Files:|Lines:|Nodes:"

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.

Build Performance Patterns

• Enable skipLibCheck: true for library type checking only (often significantly improves performance on large projects, but avoid masking app typing issues)
• Use incremental: true with .tsbuildinfo cache
• Configure include/exclude precisely
• For monorepos: Use project references with composite: true

Real-World Problem Resolution

Complex Error Patterns

"The inferred type of X cannot be named"

• Cause: Missing type export or circular dependency
• Fix priority: export the required type explicitly; use ReturnType<typeof function> helper; break circular dependencies with type-only imports
• Resource: https://github.com/microsoft/TypeScript/issues/47663

Missing type declarations — add an ambient .d.ts module declaration for untyped packages. See references/typescript-cheatsheet.md (§ Module Declarations). For more detail: Declaration Files Guide

"Excessive stack depth comparing types"

• Cause: Circular or deeply recursive types
• Fix priority: limit recursion depth with conditional types; use interface extends instead of type intersection; simplify generic constraints

// Bad: Infinite recursion
type InfiniteArray<T> = T | InfiniteArray<T>[];

// Good: Limited recursion
type NestedArray<T, D extends number = 5> =
  D extends 0 ? T : T | NestedArray<T, [-1, 0, 1, 2, 3, 4][D]>[];

Module Resolution Mysteries — "Cannot find module" despite the file existing:

1. Check moduleResolution matches your bundler
2. Verify baseUrl and paths alignment
3. For monorepos: Ensure workspace protocol (workspace:*)
4. Try clearing cache: rm -rf node_modules/.cache .tsbuildinfo

Path Mapping at Runtime — TypeScript paths only work at compile time, not runtime:

• ts-node: use ts-node -r tsconfig-paths/register
• Node ESM: use loader alternatives or avoid TS paths at runtime
• Production: pre-compile with resolved paths

Migration Expertise

JavaScript to TypeScript Migration — incremental strategy: enable allowJs/checkJs in the existing tsconfig, rename files gradually (.js → .ts), add types file by file, enable strict mode features one by one. See references/full-guide.md (§ Migration Playbook) for the full command sequence and optional automated helpers (ts-migrate, typesync).

Tool Migration Decisions

Monorepo Management

Nx vs Turborepo Decision Matrix

• Choose Turborepo if: Simple structure, need speed, <20 packages
• Choose Nx if: Complex dependencies, need visualization, plugins required
• Performance: Nx often performs better on large monorepos (>50 packages)

TypeScript Monorepo Configuration — root tsconfig references array plus composite/declaration/declarationMap per package. See references/full-guide.md (§ Monorepo TypeScript Configuration) for the full config.

Modern Tooling Expertise

Biome vs ESLint

Use Biome when: speed is critical, want a single tool for lint + format, TypeScript-first project, okay 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 (Biome doesn't have this yet).

Type Testing Strategies

Vitest Type Testing (Recommended) — write .test-d.ts files using expectTypeOf to assert on prop/return types at compile time. See references/full-guide.md (§ Vitest Type Testing) for a full example.

When to Test Types: publishing libraries, complex generic functions, type-level utilities, API contracts.

Debugging Mastery

CLI Debugging Tools — trace module resolution (tsc --traceResolution), profile type-check performance (tsc --generateTrace), debug files directly with tsx/ts-node --inspect-brk. See references/full-guide.md (§ CLI Debugging Tools) for the full command set.

Custom Error Classes — extend Error, set this.name, and call Error.captureStackTrace to preserve the stack. See references/full-guide.md (§ Custom Error Classes) for the full pattern.

Current Best Practices

Strict by Default

Use strict: true plus noUncheckedIndexedAccess, noImplicitOverride, exactOptionalPropertyTypes, noPropertyAccessFromIndexSignature. See references/tsconfig-strict.json for a full production-ready strict config.

ESM-First Approach

• Set "type": "module" in package.json
• Use .mts for TypeScript ESM files if needed
• Configure "moduleResolution": "bundler" for modern tools
• Use dynamic imports for CJS: const pkg = await import('cjs-package')
  • Note: await import() requires async function or top-level await in ESM
  • For CJS packages in ESM: may need (await import('pkg')).default depending on the package's export structure and compiler settings

AI-Assisted Development

• AI coding assistants excel at TypeScript generics and boilerplate type definitions
• Validate AI-generated types with type tests
• Document complex types for AI context

Code Review Checklist

Type Safety

• No implicit any types (use unknown or proper types)
• Strict null checks enabled and properly 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 (better error messages)
• Use const assertions for literal types
• Leverage type guards and predicates
• Avoid type gymnastics when simpler solution exists
• 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 use of barrel exports (avoid over-bundling)
• 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 + linting speed critical? → Biome
Type checking + comprehensive linting? → ESLint + typescript-eslint
Type testing? → Vitest expectTypeOf
Build tool? → Project size <10 packages? Turborepo. Else? 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

Expert Resources

• Performance: TypeScript Wiki Performance, Type instantiation tracking
• Advanced patterns: Type Challenges, Type-Level TypeScript Course
• Tools: Biome (linter/formatter), TypeStat (auto-fix types), ts-migrate (migration toolkit)
• Testing: Vitest Type Testing, tsd (standalone type testing)

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

Typescript Best Practices procedure

Read typescript-best-practices procedure when running this workflow. Apply the authorized scope and mode of this entry point to every step. Resolve other skills through this distribution’s active catalog; resolve resources relative to the installed skill directory.