Source: wshobson/agents Original Plugin: javascript-typescript

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

Use unknown over any: Enforce type checking
Prefer interface for object shapes: Better error messages
Use type for unions and complex types: More flexible
Leverage type inference: Let TypeScript infer when possible
Create helper types: Build reusable type utilities
Use const assertions: Preserve literal types
Avoid type assertions: Use type guards instead
Document complex types: Add JSDoc comments
Use strict mode: Enable all strict compiler options
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

Over-using any: Defeats the purpose of TypeScript
Ignoring strict null checks: Can lead to runtime errors
Too complex types: Can slow down compilation
Not using discriminated unions: Misses type narrowing opportunities
Forgetting readonly modifiers: Allows unintended mutations
Circular type references: Can cause compiler errors
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