50 AI Prompts for TypeScript Advanced Patterns and Type System

How to Use These TypeScript Prompts

Generics, Constraints, and Inference (Prompts 1-15)

• Prompt 4: Write a generic memoize function that preserves the argument types and return type of the wrapped function exactly. The memoized function should have the same signature as the original. Show how to use Parameters<T> and ReturnType<T> to extract and reapply types.
• Prompt 5: Create a generic EventEmitter class in TypeScript with strongly typed events. Define an EventMap type parameter that maps event names to payload types, so emitter.emit("userCreated", { id: 1 }) is type-safe and emitter.on("userCreated", (payload) => ...) infers the correct payload type.
• Prompt 6: Write a TypeScript generic that creates a validated form schema. Define FormSchema<T> where T is an object type, which produces a schema with the same keys as T but each value is a validator object with a validate(value: unknown): value is T[K] type guard method.
• Prompt 7: Create a TypeScript function overload set for a fetch helper that accepts a URL string and returns different types based on a generic parameter. fetch<User>("/api/user") returns Promise<User>, fetch<User[]>("/api/users") returns Promise<User[]>, and the function includes runtime validation using a type guard predicate passed as an argument.
• Prompt 8: Write a TypeScript type for a deeply nested path accessor. Given a type like { user: { profile: { name: string } } }, define a type Paths<T> that produces a union of dot-notation path strings: "user" | "user.profile" | "user.profile.name". Then write a get<T, P extends Paths<T>>(obj: T, path: P) function that returns the correct type for each path.
• Prompt 9: Create a TypeScript builder pattern for an HTTP request object. RequestBuilder starts with no properties and each method (withUrl, withMethod, withBody, withHeaders) returns a new type that adds that property to the accumulated type. The build() method is only available when url and method are both present, enforced by TypeScript.
• Prompt 10: Write a TypeScript type-safe dependency injection container. Define a Container<T extends Record<string, unknown>> that stores registered services, a register<K extends string, V>(key: K, value: V) method that returns a Container with the new key added to its type, and a get<K extends keyof T>(key: K) method that returns T[K].
• Prompt 11: Create a TypeScript function that accepts a union type and returns type predicates for each member. Given type Shape = Circle | Square | Triangle, generate isCircle, isSquare, and isTriangle type guard functions that TypeScript uses to narrow the type in conditional branches.
• Prompt 12: Write a TypeScript generic that produces a strict version of a function type that does not allow extra properties on object arguments. StrictArgs<T extends (...args: any[]) => any> should produce a version of T where object arguments are checked for excess properties even in indirect call contexts.
• Prompt 13: Create a TypeScript template literal type that generates API endpoint strings from a route definition object. Given { users: ["list", "create", "delete"] }, produce the union "/users/list" | "/users/create" | "/users/delete" as a type so that only valid endpoint strings are accepted by the API client.
• Prompt 14: Write a TypeScript conditional type that unwraps nested Promise types. UnwrapPromise<Promise<Promise<string>>> should resolve to string. UnwrapPromise<string> should stay as string. Show how to use the infer keyword recursively to unwrap multiple levels.
• Prompt 15: Create a TypeScript mapped type called Nullable<T> that makes all properties of T nullable (T[K] | null) and a corresponding NonNullableDeep<T> that removes null and undefined from all properties recursively. Show both definitions and test them with a deeply nested interface.

Discriminated Unions, Branded Types, and Advanced Patterns (Prompts 16-35)

• Prompt 19: Create a TypeScript read-only event system where subscribers cannot mutate the event payload. Use Readonly<T> on the event payload type in the subscriber callback, and show how this prevents accidental mutation that could affect other subscribers.
• Prompt 20: Write a TypeScript pattern for a type-safe environment variable configuration. Define an EnvSchema object that maps environment variable names to their types and required/optional status. Generate a fully typed config object at startup that validates all required variables are present and converts types (string to number for PORT).
• Prompt 21: Create a TypeScript type-safe SQL query builder for a simple SELECT statement. Define a QueryBuilder<T> generic where T is the table row type. Methods .select("name", "email") should return a builder whose result type is Pick<T, "name" | "email"> rather than T. Chain .where() and .limit() maintaining the accumulated type.
• Prompt 22: Write TypeScript utility types for an API client that automatically maps route definitions to typed methods. Given a routes definition object where each key has a request type and response type, generate a client object type where each method accepts the request type and returns Promise<ResponseType>.
• Prompt 23: Create a TypeScript Lens type for immutable nested updates. Define Lens<S, A> with get(source: S): A and set(value: A, source: S): S methods. Show how to compose two lenses to focus on a deeply nested field and how TypeScript infers the intermediate types.
• Prompt 24: Write a TypeScript module augmentation for a third-party library. Augment the express Request interface to add a currentUser property of type AuthenticatedUser. Show the declaration merging in a custom types file, how to import it globally, and how TypeScript then recognises req.currentUser in all Express route handlers.
• Prompt 25: Create TypeScript decorator factories for class method validation. Define a @MinLength(n) parameter decorator that adds runtime validation and a @LogCalls decorator that logs method invocations. Show how to use TypeScript 5 decorator metadata to make decorators type-safe.

Type System Utilities and Tooling (Prompts 36-50)

• Prompt 39: Create custom ESLint rules for a TypeScript project that enforce: no use of any in public function signatures, no unhandled Promise<void> returns, and that all exported functions have explicit return types. Show the ESLint config using @typescript-eslint.
• Prompt 40: Write a type test file using expect-type that verifies the correct types of a generic utility function. Use expectTypeOf().toEqualTypeOf() to assert the output types for different input types and catch type regressions in CI.
• Prompt 41: Create a TypeScript AST transform using ts-morph that finds all arrow functions without explicit return types and adds the inferred return type as an explicit annotation. Show running the transform as a one-off script across a codebase.
• Prompt 42: Write a TypeScript-first API contract using Zod schemas. Define schemas for request bodies and response shapes, infer TypeScript types from the schemas, use them to validate incoming requests in an Express route, and share the same schema types with the frontend for full end-to-end type safety.
• Prompt 43: Create a TypeScript type-safe feature flag system. Define a FeatureFlags record of flag names to boolean values, write a useFlag<K extends keyof FeatureFlags>(flag: K): FeatureFlags[K] hook, and ensure that accessing an undefined flag name is a compile-time error.
• Prompt 44: Write a TypeScript-first repository pattern with generic CRUD types. Define Repository<T, ID> with typed findById, findAll, save, and delete methods. Show concrete implementations for a UserRepository extending the base and how TypeScript enforces the correct ID type per entity.
• Prompt 45: Create a TypeScript const assertion pattern for configuration objects. Show how to use as const on a routes definition object to preserve literal string types, how this enables exhaustive switch statements over route names, and how it prevents accidental mutation at the type level.
• Prompt 46: Write a TypeScript satisfies operator pattern for strongly typing configuration objects without widening the type. Show the difference between using satisfies, type assertion, and direct type annotation, and when satisfies is the correct tool.
• Prompt 47: Create a TypeScript opaque type pattern using symbols for values that should not mix even if they share an underlying primitive type. Show how to define OpaqueType<T, Symbol>, create CentsDollarAmount and USDollarAmount as distinct types, and prevent accidentally adding them together.
• Prompt 48: Write a TypeScript HKT (Higher-Kinded Type) simulation using interface merging and type maps. Define a Functor interface with a map operation, implement it for Array and Option types, and write a generic function that works on any Functor without concrete type knowledge.
• Prompt 49: Create a TypeScript inference test for a complex generic type. Show how to use conditional type debugging tricks — wrapping in a tuple to prevent distribution, using infer in surprising places, using the NoInfer utility type — to get TypeScript to infer the type you expect.
• Prompt 50: Write a TypeScript migration guide and codemods for upgrading a large JavaScript codebase. Use ts-morph to add JSDoc type annotations to all functions, generate a tsconfig.json with allowJs and incremental strict mode, add explicit any annotations where types cannot be inferred, and produce a migration progress report showing files converted versus remaining.