TypeScript Core Patterns Modern TypeScript development patterns for type safety, runtime validation, and optimal configuration. Quick Start New Project: Use 2025 tsconfig → Enable strict + noUncheckedIndexedAccess → Choose Zod for validation Existing Project: Enable strict: false initially → Fix any with unknown → Add noUncheckedIndexedAccess API Development: Zod schemas at boundaries → z.infer<typeof Schema for types → satisfies for routes Library Development: Enable declaration: true → Use const type parameters → See [advanced patterns 2025.md](./references/advanced patterns 2025.md) Quick Reference tsconfig.json 2025 Baseline Key Compiler Options Option Purpose When to Enable noUncheckedIndexedAccess Forces null checks on array/object access Always for safety exactOptionalPropertyTypes Distinguishes undefined from missing APIs with optional fields verbatimModuleSyntax Enforces explicit type only imports ESM projects erasableSyntaxOnly Node.js 22+ native TS support Type stripping environments Local Baselines See references/configuration.md for repo specific tsconfig patterns (CommonJS CLI, NodeNext strict, Next.js bundler). Core Type Patterns Const Type Parameters Preserve literal types through generic functions: Satisfies Operator Validate against a type while preserving literal inference: Template Literal Types Type safe string manipulation and route extraction: Discriminated Unions with Exhaustiveness Runtime Validation TypeScript types disappear at runtime. Use validation libraries for external data (APIs, forms, config files). Quick Comparison Library Bundle Size Speed Best For Zod ~13.5kB Baseline Full stack apps, tRPC integration TypeBox ~8kB ~10x faster OpenAPI, performance critical Valibot ~1.4kB ~2x faster Edge functions, minimal bundles Basic Pattern (Zod) → See [runtime validation.md](./references/runtime validation.md) for complete Zod, TypeBox, and Valibot patterns Decision Support Quick Decision Guide Need to choose between type vs interface ? Public API / library types → interface Union types / mapped types → type Simple object shapes → interface (default) Need generics or union types? Output type depends on input type → Generics Fixed set of known types → Union types Building reusable data structures → Generics Dealing with unknown data? External data (API, user input) → unknown (type safe) Rapid prototyping / migration → any (temporarily) Need runtime validation? Full stack TypeScript with tRPC → Zod OpenAPI / high performance → TypeBox Edge functions / minimal bundle → Valibot → See [decision trees.md](./references/decision trees.md) for comprehensive decision frameworks Troubleshooting Common Issues Quick Reference Property does not exist on type → Define proper interface or use optional properties Type is not assignable → Fix property types or use runtime validation (Zod) Object is possibly 'undefined' → Use optional chaining ( ?. ) or type guards Cannot find module → Check file extensions (.js for ESM) and module resolution Slow compilation → Enable incremental , use skipLibCheck , consider esbuild/swc → See [troubleshooting.md](./references/troubleshooting.md) for detailed solutions with examples Navigation Detailed References [📐 Advanced Types](./references/advanced types.md) Conditional types, mapped types, infer keyword, recursive types. Load when building complex type utilities or generic libraries. [⚙️ Configuration](./references/configuration.md) Complete tsconfig.json guide, project references, monorepo patterns. Load when setting up new projects or optimizing builds. [🔒 Runtime Validation](./references/runtime validation.md) Zod, TypeBox, Valibot deep patterns, error handling, integration strategies. Load when implementing API validation or form handling. [✨ Advanced Patterns 2025](./references/advanced patterns 2025.md) TypeScript 5.2+ features: using keyword, stable decorators, import type behavior, satisfies with generics. Load when using modern language features. [🌳 Decision Trees](./references/decision trees.md) Clear decision frameworks for type vs interface , generics vs unions, unknown vs any , validation library selection, type narrowing strategies, and module resolution. Load when making TypeScript design decisions. [🔧 Troubleshooting](./references/troubleshooting.md) Common TypeScript errors and fixes, type inference issues, module resolution problems, tsconfig misconfigurations, build performance optimization, and type compatibility errors. Load when debugging TypeScript issues. JavaScript / Runtime Quality Anti Patterns The type system catches type errors, but a class of JavaScript defects is runtime/AST level and survives into emitted JS and plain JS files. Watch for: Loose equality ( == / != ) — coercion bugs and auth flaws; use === / !== (accept == null only when commented as "null or undefined"). Dynamic code execution ( eval , new Function(str) , string setTimeout ) — injection risk and unnecessary; use direct syntax. Mutating builtins ( Object / Array / Function.prototype ) — pollutes for…in , breaks the whole runtime; extend or use free functions instead. Variable shadowing , var instead of let / const , using functions before declaration — readability and "wrong variable" bugs. Logical OR in switch case labels ( case 1 2: only matches 1) — use stacked case labels. Repetitive deep member access — cache the resolved chain in a local (esp. DOM). Non wrapped IIFEs , backslash multiline strings , new Array() — readability and well known traps; prefer wrapped IIFEs, template literals, and array literals. See [JS/TS Quality Anti Patterns](./references/js quality antipatterns.md) for each defect with compliant/non compliant examples, severities, false positive filters, and the equivalent ESLint/SonarSource rule. Derived from CAST Highlight JavaScript code quality indicators (https://doc.casthighlight.com/), cross referenced to ESLint core rules and SonarSource RSPEC. Red Flags Stop and reconsider if: Using any instead of unknown for external data Casting with as without runtime validation Disabling strict mode for convenience Using @ts ignore without clear justification Index access without noUncheckedIndexedAccess Integration with Other Skills nextjs core : Type safe Server Actions and route handlers nextjs v16 : Async API patterns and Cache Components typing mcp builder : Zod schemas for MCP tool inputs Related Skills When using Core, these skills enhance your workflow: react : TypeScript with React: component typing, hooks, generics nextjs : TypeScript in Next.js: Server Components, Server Actions typing drizzle : Type safe database queries with Drizzle ORM prisma : Prisma's generated TypeScript types for database schemas [Full documentation available in these skills if deployed in your bundle]

typescript-core