TypeScript Practice

Master the type system, generics, mapped types, and runtime safety patterns.

Language Essentials

Concept	Details
Memory Model	V8 GC, prototypal inheritance, closures, WeakRef/FinalizationRegistry
Type System	Structural, gradual, mapped types, conditional types, template literals
Error Handling	try/catch (untyped), Result<T,E> pattern, never for exhaustiveness
Concurrency	Event loop, Promise, async/await, Web Workers, SharedArrayBuffer
Build	tsc, tsx, esbuild, vitest for testing
Standard	TypeScript 5.x, ES2023+ target

Key Idioms

• Discriminated unions with kind field for exhaustive switching
• satisfies operator for type checking without widening
• Branded types for nominal typing (type USD = number & { __brand: 'USD' })
• as const for literal type inference
• Mapped types (Partial, Required, Pick, custom)
• Conditional types with infer for type-level computation
• Template literal types for string pattern enforcement
• using keyword (TC39 explicit resource management)

Reference Documentation

• TypeScript Handbook — free, official
• TypeScript Playground — experiment online
• type-challenges — free, type gymnastics
• Programming TypeScript (Cherny) — comprehensive
• Effective TypeScript (Vanderkam, 2nd ed.) — 83 practical items

Practising these

practice/bin/ol list  build typescript          # which of these have references yet
practice/bin/ol start build typescript 01       # clone it stubbed into .scratchpad/
practice/bin/ol check build typescript 01       # run it; exit code is the verdict
practice/bin/ol diff  build typescript 01       # your attempt against the reference

Exercises are ordered by concept, not by difficulty, so pick by the stars rather than by the number. An exercise listed below with no reference yet is a row in this table and nothing more; see the practice path for how the harness works.

Exercises

#	Exercise	Concepts	Difficulty
01	Type-safe event emitter	Generics, mapped types, Parameters<T>	⭐⭐⭐
02	Result/Option monads	Discriminated unions, method chaining, never	⭐⭐
03	Schema validator (like Zod)	Template literals, recursive types, infer	⭐⭐⭐⭐⭐
04	Reactive signals (like Solid)	Proxies, dependency tracking, batched updates	⭐⭐⭐⭐
05	Type-safe SQL query builder	Template literal types, conditional types	⭐⭐⭐⭐
06	Middleware pipeline (like Koa)	Generics, async composition, context threading	⭐⭐⭐
07	State machine (typed transitions)	Mapped types, phantom states, never transitions	⭐⭐⭐⭐
08	DI container (type-safe)	Symbol keys, Map, WeakMap, interface registry	⭐⭐⭐
09	Incremental type checker (subset)	AST, inference, unification, variance	⭐⭐⭐⭐⭐
10	Streaming JSON parser	AsyncGenerator, backpressure, ReadableStream	⭐⭐⭐⭐