Master TypeScript for Robust Code

This skill provides advanced, practical TypeScript expertise spanning type-level programming (branded/nominal types to prevent primitive obsession, recursive conditional types like DeepReadonly<T> with depth-limiting to avoid instantiation errors, template literal type patterns, satisfies for constraint validation without losing literal types, as const for maximum inference), performance diagnosis (tsc --extendedDiagnostics/--generateTrace for slow type checking, skipLibCheck/incremental/project references as fixes, common causes of "Type instantiation is excessively deep"), and real-world error resolution (missing type exports, circular dependencies via type-only imports, ambient .d.ts declarations for untyped packages, module resolution mysteries, and the fact that TypeScript path mapping only works at compile time, not runtime).

It covers incremental JavaScript-to-TypeScript migration (allowJs/checkJs first, file-by-file renaming, ts-migrate/typesync tooling), concrete tool-migration decision tables (ESLint+Prettier to Biome, Lerna to Nx/Turborepo, CJS to ESM), a Nx-vs-Turborepo decision matrix (Turborepo for <20 packages needing speed, Nx for complex/large monorepos), TypeScript project-references configuration for monorepos, a Biome-vs-ESLint tradeoff (Biome faster but no type-aware linting yet), Vitest-based type testing (expectTypeOf) for libraries and generic APIs, and a comprehensive code review checklist across type safety, TypeScript idioms, performance, module system, error handling, and code organization. It opens by explicitly deferring to narrower experts when needed - bundler internals to typescript-build-expert, ESM/CJS migration and circular dependency analysis to typescript-module-expert, and type-performance/compiler-internals profiling to typescript-type-expert - and follows a fixed workflow: detect the project's tooling/monorepo setup first (preferring Read/Grep/Glob over shell), apply the right strategy, then validate with fast one-shot typecheck/test/build commands rather than long-lived watch processes.

Use this skill for type-level programming problems, TypeScript performance issues (slow compilation, excessive type depth), real-world compiler error debugging, JavaScript-to-TypeScript migration planning, monorepo tooling decisions, or a structured TypeScript code review.

Do not use it for ultra-specific bundler internals, deep ESM/CJS circular-dependency untangling, or compiler-internals type-performance profiling - the skill explicitly hands those off to its three sibling experts rather than attempting them itself.

Input is a TypeScript codebase, error message, performance complaint, or migration/tooling decision. Output follows a fixed loop: detect project setup (TS/Node versions, tooling, monorepo structure), identify the problem category, apply the matching solution (a type pattern, a config fix, a migration step), then validate with typecheck/test/build commands run once rather than in watch mode.

npx tsc --extendedDiagnostics --incremental false

Works across the TypeScript compiler (tsc), Vitest for type testing, Biome and ESLint/typescript-eslint for linting, Nx and Turborepo for monorepo builds, and migration tools ts-migrate/typesync. Delegates to typescript-build-expert, typescript-module-expert, and typescript-type-expert for narrower expertise.

TypeScript developers debugging complex type errors or performance issues, migrating JavaScript codebases, choosing between modern tooling (Biome/ESLint, Nx/Turborepo), or conducting a thorough TypeScript-specific code review.