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.

Quick Reference

See references/best-practices.md for practical guidelines on:

• Configuration: Essential tsconfig.json settings (strict, noUncheckedIndexedAccess, exactOptionalPropertyTypes)
• Type Inference vs Explicit Types: When to annotate vs let TypeScript infer
• Interfaces vs Types: Strategic guidelines for choosing between them
• Module/Path Mapping: tsconfig paths and barrel exports patterns
• Error Handling: Custom error classes and Result pattern
• Performance: Type system performance considerations
• Type Testing: Expect<Equal<X, Y>> patterns for compile-time type tests

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:

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:

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:

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:

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

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

Extracting Return Types:

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:

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

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

Nested Conditions:

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:

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:

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

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

Key Remapping:

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:

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:

type EventName = "click" | "focus" | "blur";
type EventHandler = `on${Capitalize<EventName>}`;
// Type: "onClick" | "onFocus" | "onBlur"

String Manipulation:

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

Path Building:

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:

// 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

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

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

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

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

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

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

// 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

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

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

For detailed configuration and patterns, see references/best-practices.md.

Core Guidelines

1. Use unknown over any: Enforce type checking
2. Prefer interface for object shapes: Better error messages, declaration merging
3. Use type for unions and complex types: More flexible for computed types
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 Strategy Summary

Scenario	Recommendation
Function parameters	Always explicit
Return types	Let TypeScript infer (usually)
Local variables	Let TypeScript infer
Public API boundaries	Always explicit
Object shapes (extensible)	Use interface
Union types	Use type
Computed/mapped types	Use type

Recommended tsconfig.json Settings

{
  "compilerOptions": {
    "strict": true,
    "noUncheckedIndexedAccess": true,
    "exactOptionalPropertyTypes": true,
    "moduleResolution": "Bundler"
  }
}

Type Testing

For detailed patterns, see references/best-practices.md.

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

// More robust equality check (handles edge cases better)
type Expect<T extends true> = T;
type Equal<X, Y> = (<T>() => T extends X ? 1 : 2) extends <T>() => T extends Y ? 1 : 2
  ? true
  : false;

// Usage examples
type Test1 = Expect<Equal<string, string>>; // passes
type Test2 = Expect<Equal<string, number>>; // fails at compile time

// Test type behavior at compile time
type TestReturnType = Expect<
  Equal<ReturnType<typeof myFunction>, ExpectedType>
>;

// 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

For detailed examples, see references/best-practices.md.

• 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
• Prefer type guards over type assertions for runtime safety
• Keep union types small and simple in hot paths

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