TypeScript — Literal Types

Utility types

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

type PartialUser = Partial<User>;
type RequiredUser = Required<User>;
type UserBasic = Pick<User, 'name' | 'email'>;
type UserWithoutEmail = Omit<User, 'email'>;
type UserMap = Record<string, User>;

Mapped types

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

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

type Nullable<T> = {
  [K in keyof T]: T[K] | null;
};

Conditional types

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

type A = IsString<'hello'>; // true
type B = IsString<42>;      // false

Infer keyword

type ElementType<T> = T extends (infer U)[] ? U : T;

type X = ElementType<number[]>; // number
type Y = ElementType<string>;   // string

Template literal types

type EventName = `on${Capitalize<'click' | 'hover'>}`;
// "onClick" | "onHover"

Mini Practice

Write TypeScript code that:

1. Uses utility types (Partial, Pick, Omit)
2. Creates a mapped type
3. Uses conditional types
4. Demonstrates template literal types

Up Next

In the next lesson, you'll learn about Decorators — class and method decorators.