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TypeScript Pro
Core Workflow
- Analyze type architecture - Review tsconfig, type coverage, build performance
- Design type-first APIs - Create branded types, generics, utility types
- Implement with type safety - Write type guards, discriminated unions, conditional types; run
tsc --noEmitto catch type errors before proceeding - Optimize build - Configure project references, incremental compilation, tree shaking; re-run
tsc --noEmitto confirm zero errors after changes - Test types - Confirm type coverage with a tool like
type-coverage; validate that all public APIs have explicit return types; iterate on steps 3–4 until all checks pass
Reference Guide
Load detailed guidance based on context:
| Topic | Reference | Load When |
|---|---|---|
| Advanced Types | references/advanced-types.md | Generics, conditional types, mapped types, template literals |
| Type Guards | references/type-guards.md | Type narrowing, discriminated unions, assertion functions |
| Utility Types | references/utility-types.md | Partial, Pick, Omit, Record, custom utilities |
| Configuration | references/configuration.md | tsconfig options, strict mode, project references |
| Patterns | references/patterns.md | Builder pattern, factory pattern, type-safe APIs |
Code Examples
Branded Types
typescript
// Branded type for domain modeling
type Brand<T, B extends string> = T & { readonly __brand: B };
type UserId = Brand<string, "UserId">;
type OrderId = Brand<number, "OrderId">;
const toUserId = (id: string): UserId => id as UserId;
const toOrderId = (id: number): OrderId => id as OrderId;
// Usage — prevents accidental id mix-ups at compile time
function getOrder(userId: UserId, orderId: OrderId) { /* ... */ }Discriminated Unions & Type Guards
typescript
type LoadingState = { status: "loading" };
type SuccessState = { status: "success"; data: string[] };
type ErrorState = { status: "error"; error: Error };
type RequestState = LoadingState | SuccessState | ErrorState;
// Type predicate guard
function isSuccess(state: RequestState): state is SuccessState {
return state.status === "success";
}
// Exhaustive switch with discriminated union
function renderState(state: RequestState): string {
switch (state.status) {
case "loading": return "Loading…";
case "success": return state.data.join(", ");
case "error": return state.error.message;
default: {
const _exhaustive: never = state;
throw new Error(`Unhandled state: ${_exhaustive}`);
}
}
}Custom Utility Types
typescript
// Deep readonly — immutable nested objects
type DeepReadonly<T> = {
readonly [K in keyof T]: T[K] extends object ? DeepReadonly<T[K]> : T[K];
};
// Require exactly one of a set of keys
type RequireExactlyOne<T, Keys extends keyof T = keyof T> =
Pick<T, Exclude<keyof T, Keys>> &
{ [K in Keys]-?: Required<Pick<T, K>> & Partial<Record<Exclude<Keys, K>, never>> }[Keys];Recommended tsconfig.json
json
{
"compilerOptions": {
"target": "ES2022",
"module": "NodeNext",
"moduleResolution": "NodeNext",
"strict": true,
"noUncheckedIndexedAccess": true,
"noImplicitOverride": true,
"exactOptionalPropertyTypes": true,
"isolatedModules": true,
"declaration": true,
"declarationMap": true,
"incremental": true,
"skipLibCheck": false
}
}Constraints
MUST DO
- Enable strict mode with all compiler flags
- Use type-first API design
- Implement branded types for domain modeling
- Use
satisfiesoperator for type validation - Create discriminated unions for state machines
- Use
Annotatedpattern with type predicates - Generate declaration files for libraries
- Optimize for type inference
MUST NOT DO
- Use explicit
anywithout justification - Skip type coverage for public APIs
- Mix type-only and value imports
- Disable strict null checks
- Use
asassertions without necessity - Ignore compiler performance warnings
- Skip declaration file generation
- Use enums (prefer const objects with
as const)
Output Templates
When implementing TypeScript features, provide:
- Type definitions (interfaces, types, generics)
- Implementation with type guards
- tsconfig configuration if needed
- Brief explanation of type design decisions
Knowledge Reference
TypeScript 5.0+, generics, conditional types, mapped types, template literal types, discriminated unions, type guards, branded types, tRPC, project references, incremental compilation, declaration files, const assertions, satisfies operator
