typescript-advanced-types

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Master TypeScript's advanced type system including generics, conditional types, mapped types, template literals, and utility types for building type-safe applications. Use when implementing complex type logic, creating reusable type utilities, or ensuring compile-time type safety in TypeScript projects.

ID: typescript-advanced-types

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TypeScript Advanced Types

Comprehensive guidance for mastering TypeScript's advanced type system including generics, conditional types, mapped types, template literal types, and utility types for building robust, type-safe applications.

When to Use This Skill

Building type-safe libraries or frameworks
Creating reusable generic components
Implementing complex type inference logic
Designing type-safe API clients
Building form validation systems
Creating strongly-typed configuration objects
Implementing type-safe state management
Migrating JavaScript codebases to TypeScript

Core Concepts

1. Generics

Purpose: Create reusable, type-flexible components while maintaining type safety.

Basic Generic Function:

``typescript


function identity<T>(value: T): T {
  return value;
}


const num = identity<number>(42);        // Type: number
const str = identity<string>("hello");    // Type: string
const auto = identity(true);              // Type inferred: boolean




Generic Constraints:

`typescript


interface HasLength {
  length: number;
}


function logLength<T extends HasLength>(item: T): T {
  console.log(item.length);
  return item;
}


logLength("hello");           // OK: string has length
logLength([1, 2, 3]);         // OK: array has length
logLength({ length: 10 });    // OK: object has length
// logLength(42);             // Error: number has no length




Multiple Type Parameters:

`typescript


function merge<T, U>(obj1: T, obj2: U): T & U {
  return { ...obj1, ...obj2 };
}


const merged = merge(
  { name: "John" },
  { age: 30 }
);
// Type: { name: string } & { age: number }




2. Conditional Types


Purpose: Create types that depend on conditions, enabling sophisticated type logic.


Basic Conditional Type:

`typescript


type IsString<T> = T extends string ? true : false;


type A = IsString<string>;    // true
type B = IsString<number>;    // false




Extracting Return Types:

`typescript


type ReturnType<T> = T extends (...args: any[]) => infer R ? R : never;


function getUser() {
  return { id: 1, name: "John" };
}


type User = ReturnType<typeof getUser>;
// Type: { id: number; name: string; }




Distributive Conditional Types:

`typescript


type ToArray<T> = T extends any ? T[] : never;


type StrOrNumArray = ToArray<string | number>;
// Type: string[] | number[]




Nested Conditions:

`typescript


type TypeName<T> =
  T extends string ? "string" :
  T extends number ? "number" :
  T extends boolean ? "boolean" :
  T extends undefined ? "undefined" :
  T extends Function ? "function" :
  "object";


type T1 = TypeName<string>;     // "string"
type T2 = TypeName<() => void>; // "function"




3. Mapped Types


Purpose: Transform existing types by iterating over their properties.


Basic Mapped Type:

`typescript


type Readonly<T> = {
  readonly [P in keyof T]: T[P];
};


interface User {
  id: number;
  name: string;
}


type ReadonlyUser = Readonly<User>;
// Type: { readonly id: number; readonly name: string; }




Optional Properties:

`typescript


type Partial<T> = {
  [P in keyof T]?: T[P];
};


type PartialUser = Partial<User>;
// Type: { id?: number; name?: string; }




Key Remapping:

`typescript


type Getters<T> = {

[K in keyof T as get${Capitalize<string & K>}]: () => T[K]


};


interface Person {
  name: string;
  age: number;
}


type PersonGetters = Getters<Person>;
// Type: { getName: () => string; getAge: () => number; }




Filtering Properties:

`typescript


type PickByType<T, U> = {
  [K in keyof T as T[K] extends U ? K : never]: T[K]
};


interface Mixed {
  id: number;
  name: string;
  age: number;
  active: boolean;
}


type OnlyNumbers = PickByType<Mixed, number>;
// Type: { id: number; age: number; }




4. Template Literal Types


Purpose: Create string-based types with pattern matching and transformation.


Basic Template Literal:

`typescript


type EventName = "click" | "focus" | "blur";

type EventHandler = on${Capitalize<EventName>};


// Type: "onClick" | "onFocus" | "onBlur"




String Manipulation:

`typescript


type UppercaseGreeting = Uppercase<"hello">;  // "HELLO"
type LowercaseGreeting = Lowercase<"HELLO">;  // "hello"
type CapitalizedName = Capitalize<"john">;    // "John"
type UncapitalizedName = Uncapitalize<"John">; // "john"




Path Building:

`typescript


type Path<T> = T extends object
  ? { [K in keyof T]: K extends string

? ${K} | ${K}.${Path<T[K]>}


      : never
    }[keyof T]
  : never;


interface Config {
  server: {
    host: string;
    port: number;
  };
  database: {
    url: string;
  };
}


type ConfigPath = Path<Config>;
// Type: "server" | "database" | "server.host" | "server.port" | "database.url"




5. Utility Types


Built-in Utility Types:

`typescript


// Partial<T> - Make all properties optional
type PartialUser = Partial<User>;


// Required<T> - Make all properties required
type RequiredUser = Required<PartialUser>;


// Readonly<T> - Make all properties readonly
type ReadonlyUser = Readonly<User>;


// Pick<T, K> - Select specific properties
type UserName = Pick<User, "name" | "email">;


// Omit<T, K> - Remove specific properties
type UserWithoutPassword = Omit<User, "password">;


// Exclude<T, U> - Exclude types from union
type T1 = Exclude<"a" | "b" | "c", "a">;  // "b" | "c"


// Extract<T, U> - Extract types from union
type T2 = Extract<"a" | "b" | "c", "a" | "b">;  // "a" | "b"


// NonNullable<T> - Exclude null and undefined
type T3 = NonNullable<string | null | undefined>;  // string


// Record<K, T> - Create object type with keys K and values T
type PageInfo = Record<"home" | "about", { title: string }>;




Advanced Patterns


Pattern 1: Type-Safe Event Emitter

`typescript


type EventMap = {
  "user:created": { id: string; name: string };
  "user:updated": { id: string };
  "user:deleted": { id: string };
};


class TypedEventEmitter<T extends Record<string, any>> {
  private listeners: {
    [K in keyof T]?: Array<(data: T[K]) => void>;
  } = {};


  on<K extends keyof T>(event: K, callback: (data: T[K]) => void): void {
    if (!this.listeners[event]) {
      this.listeners[event] = [];
    }
    this.listeners[event]!.push(callback);
  }


  emit<K extends keyof T>(event: K, data: T[K]): void {
    const callbacks = this.listeners[event];
    if (callbacks) {
      callbacks.forEach(callback => callback(data));
    }
  }
}


const emitter = new TypedEventEmitter<EventMap>();


emitter.on("user:created", (data) => {
  console.log(data.id, data.name);  // Type-safe!
});


emitter.emit("user:created", { id: "1", name: "John" });
// emitter.emit("user:created", { id: "1" });  // Error: missing 'name'




Pattern 2: Type-Safe API Client

`typescript


type HTTPMethod = "GET" | "POST" | "PUT" | "DELETE";


type EndpointConfig = {
  "/users": {
    GET: { response: User[] };
    POST: { body: { name: string; email: string }; response: User };
  };
  "/users/:id": {
    GET: { params: { id: string }; response: User };
    PUT: { params: { id: string }; body: Partial<User>; response: User };
    DELETE: { params: { id: string }; response: void };
  };
};


type ExtractParams<T> = T extends { params: infer P } ? P : never;
type ExtractBody<T> = T extends { body: infer B } ? B : never;
type ExtractResponse<T> = T extends { response: infer R } ? R : never;


class APIClient<Config extends Record<string, Record<HTTPMethod, any>>> {
  async request<
    Path extends keyof Config,
    Method extends keyof Config[Path]
  >(
    path: Path,
    method: Method,
    ...[options]: ExtractParams<Config[Path][Method]> extends never
      ? ExtractBody<Config[Path][Method]> extends never
        ? []
        : [{ body: ExtractBody<Config[Path][Method]> }]
      : [{
          params: ExtractParams<Config[Path][Method]>;
          body?: ExtractBody<Config[Path][Method]>;
        }]
  ): Promise<ExtractResponse<Config[Path][Method]>> {
    // Implementation here
    return {} as any;
  }
}


const api = new APIClient<EndpointConfig>();


// Type-safe API calls
const users = await api.request("/users", "GET");
// Type: User[]


const newUser = await api.request("/users", "POST", {
  body: { name: "John", email: "john@example.com" }
});
// Type: User


const user = await api.request("/users/:id", "GET", {
  params: { id: "123" }
});
// Type: User




Pattern 3: Builder Pattern with Type Safety

`typescript


type BuilderState<T> = {
  [K in keyof T]: T[K] | undefined;
};


type RequiredKeys<T> = {
  [K in keyof T]-?: {} extends Pick<T, K> ? never : K;
}[keyof T];


type OptionalKeys<T> = {
  [K in keyof T]-?: {} extends Pick<T, K> ? K : never;
}[keyof T];


type IsComplete<T, S> =
  RequiredKeys<T> extends keyof S
    ? S[RequiredKeys<T>] extends undefined
      ? false
      : true
    : false;


class Builder<T, S extends BuilderState<T> = {}> {
  private state: S = {} as S;


  set<K extends keyof T>(
    key: K,
    value: T[K]
  ): Builder<T, S & Record<K, T[K]>> {
    this.state[key] = value;
    return this as any;
  }


  build(
    this: IsComplete<T, S> extends true ? this : never
  ): T {
    return this.state as T;
  }
}


interface User {
  id: string;
  name: string;
  email: string;
  age?: number;
}


const builder = new Builder<User>();


const user = builder
  .set("id", "1")
  .set("name", "John")
  .set("email", "john@example.com")
  .build();  // OK: all required fields set


// const incomplete = builder
//   .set("id", "1")
//   .build();  // Error: missing required fields




Pattern 4: Deep Readonly/Partial

`typescript


type DeepReadonly<T> = {
  readonly [P in keyof T]: T[P] extends object
    ? T[P] extends Function
      ? T[P]
      : DeepReadonly<T[P]>
    : T[P];
};


type DeepPartial<T> = {
  [P in keyof T]?: T[P] extends object
    ? T[P] extends Array<infer U>
      ? Array<DeepPartial<U>>
      : DeepPartial<T[P]>
    : T[P];
};


interface Config {
  server: {
    host: string;
    port: number;
    ssl: {
      enabled: boolean;
      cert: string;
    };
  };
  database: {
    url: string;
    pool: {
      min: number;
      max: number;
    };
  };
}


type ReadonlyConfig = DeepReadonly<Config>;
// All nested properties are readonly


type PartialConfig = DeepPartial<Config>;
// All nested properties are optional




Pattern 5: Type-Safe Form Validation

`typescript


type ValidationRule<T> = {
  validate: (value: T) => boolean;
  message: string;
};


type FieldValidation<T> = {
  [K in keyof T]?: ValidationRule<T[K]>[];
};


type ValidationErrors<T> = {
  [K in keyof T]?: string[];
};


class FormValidator<T extends Record<string, any>> {
  constructor(private rules: FieldValidation<T>) {}


  validate(data: T): ValidationErrors<T> | null {
    const errors: ValidationErrors<T> = {};
    let hasErrors = false;


    for (const key in this.rules) {
      const fieldRules = this.rules[key];
      const value = data[key];


      if (fieldRules) {
        const fieldErrors: string[] = [];


        for (const rule of fieldRules) {
          if (!rule.validate(value)) {
            fieldErrors.push(rule.message);
          }
        }


        if (fieldErrors.length > 0) {
          errors[key] = fieldErrors;
          hasErrors = true;
        }
      }
    }


    return hasErrors ? errors : null;
  }
}


interface LoginForm {
  email: string;
  password: string;
}


const validator = new FormValidator<LoginForm>({
  email: [
    {
      validate: (v) => v.includes("@"),
      message: "Email must contain @"
    },
    {
      validate: (v) => v.length > 0,
      message: "Email is required"
    }
  ],
  password: [
    {
      validate: (v) => v.length >= 8,
      message: "Password must be at least 8 characters"
    }
  ]
});


const errors = validator.validate({
  email: "invalid",
  password: "short"
});
// Type: { email?: string[]; password?: string[]; } | null




Pattern 6: Discriminated Unions

`typescript


type Success<T> = {
  status: "success";
  data: T;
};


type Error = {
  status: "error";
  error: string;
};


type Loading = {
  status: "loading";
};


type AsyncState<T> = Success<T> | Error | Loading;


function handleState<T>(state: AsyncState<T>): void {
  switch (state.status) {
    case "success":
      console.log(state.data);  // Type: T
      break;
    case "error":
      console.log(state.error);  // Type: string
      break;
    case "loading":
      console.log("Loading...");
      break;
  }
}


// Type-safe state machine
type State =
  | { type: "idle" }
  | { type: "fetching"; requestId: string }
  | { type: "success"; data: any }
  | { type: "error"; error: Error };


type Event =
  | { type: "FETCH"; requestId: string }
  | { type: "SUCCESS"; data: any }
  | { type: "ERROR"; error: Error }
  | { type: "RESET" };


function reducer(state: State, event: Event): State {
  switch (state.type) {
    case "idle":
      return event.type === "FETCH"
        ? { type: "fetching", requestId: event.requestId }
        : state;
    case "fetching":
      if (event.type === "SUCCESS") {
        return { type: "success", data: event.data };
      }
      if (event.type === "ERROR") {
        return { type: "error", error: event.error };
      }
      return state;
    case "success":
    case "error":
      return event.type === "RESET" ? { type: "idle" } : state;
  }
}




Type Inference Techniques


1. Infer Keyword

`typescript


// Extract array element type
type ElementType<T> = T extends (infer U)[] ? U : never;


type NumArray = number[];
type Num = ElementType<NumArray>;  // number


// Extract promise type
type PromiseType<T> = T extends Promise<infer U> ? U : never;


type AsyncNum = PromiseType<Promise<number>>;  // number


// Extract function parameters
type Parameters<T> = T extends (...args: infer P) => any ? P : never;


function foo(a: string, b: number) {}
type FooParams = Parameters<typeof foo>;  // [string, number]




2. Type Guards

`typescript


function isString(value: unknown): value is string {
  return typeof value === "string";
}


function isArrayOf<T>(
  value: unknown,
  guard: (item: unknown) => item is T
): value is T[] {
  return Array.isArray(value) && value.every(guard);
}


const data: unknown = ["a", "b", "c"];


if (isArrayOf(data, isString)) {
  data.forEach(s => s.toUpperCase());  // Type: string[]
}




3. Assertion Functions

`typescript


function assertIsString(value: unknown): asserts value is string {
  if (typeof value !== "string") {
    throw new Error("Not a string");
  }
}


function processValue(value: unknown) {
  assertIsString(value);
  // value is now typed as string
  console.log(value.toUpperCase());
}




Best Practices

1. Use unknown over any: Enforce type checking

2. Prefer interface for object shapes: Better error messages

3. Use type for unions and complex types: More flexible


4. Leverage type inference: Let TypeScript infer when possible
5. Create helper types: Build reusable type utilities
6. Use const assertions: Preserve literal types
7. Avoid type assertions: Use type guards instead
8. Document complex types: Add JSDoc comments
9. Use strict mode: Enable all strict compiler options
10. Test your types: Use type tests to verify type behavior


Type Testing

`typescript


// Type assertion tests
type AssertEqual<T, U> =
  [T] extends [U]
    ? [U] extends [T]
      ? true
      : false
    : false;


type Test1 = AssertEqual<string, string>;        // true
type Test2 = AssertEqual<string, number>;        // false
type Test3 = AssertEqual<string | number, string>; // false


// Expect error helper
type ExpectError<T extends never> = T;


// Example usage
type ShouldError = ExpectError<AssertEqual<string, number>>;




Common Pitfalls

1. Over-using any`: Defeats the purpose of TypeScript

2. Ignoring strict null checks: Can lead to runtime errors

3. Too complex types: Can slow down compilation

4. Not using discriminated unions: Misses type narrowing opportunities

5. Forgetting readonly modifiers: Allows unintended mutations

6. Circular type references: Can cause compiler errors

7. Not handling edge cases: Like empty arrays or null values

Performance Considerations

Avoid deeply nested conditional types
Use simple types when possible
Cache complex type computations
Limit recursion depth in recursive types
Use build tools to skip type checking in production

Resources

TypeScript Handbook: https://www.typescriptlang.org/docs/handbook/
Type Challenges: https://github.com/type-challenges/type-challenges
TypeScript Deep Dive: https://basarat.gitbook.io/typescript/
Effective TypeScript: Book by Dan Vanderkam

Informacion

Estado	Activo
Origen	upstream
Usos	3
Ultimo uso	23/01 16:57
Actualizado	09/01/2026 02:14

Archivo origen

Z:\Repositorios\core\upstream\plugins\javascript-typescript\skills\typescript-advanced-types\SKILL.md