[TS] 유틸리티 타입(Utility Types)

type A = Awaited<Promise<string>>;   
// type A = string
 
type B = Awaited<Promise<Promise<number>>>;
// type B = number
 
type C = Awaited<boolean | Promise<number>>; 
// type C = number | boolean
복사

interface CatInfo {
  age: number;
  breed: string;
}
 
type CatName = "miffy" | "boris" | "mordred";
 
const cats: Record<CatName, CatInfo> = {
  miffy: { age: 10, breed: "Persian" },
  boris: { age: 5, breed: "Maine Coon" },
  mordred: { age: 16, breed: "British Shorthair" },
};
복사

// 대상 타입
interface Todo {
  title: string;
  description: string;
  completed: boolean;
}
 
// Todo 타입에서 title, completed 속성만 뽑아 구성한 유틸리티 타입
type TodoPreview = Pick<Todo, "title" | "completed">;
 
const todo: TodoPreview = {
  title: "Clean room",
  completed: false,
};
복사

type T0 = Extract<"a" | "b" | "c", "a" | "f">;  
// type T0 = "a"
 
type T1 = Extract<string | number | (() => void), Function>;  
// type T1 = () => void
 
type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "square"; x: number }
  | { kind: "triangle"; x: number; y: number }; 
 
type T2 = Extract<Shape, { kind: "circle" }>  
// type T2 = {
//     kind: "circle";
//     radius: number;
// }
복사

// 속성 제거 대상 타입
interface Todo {
  title: string;
  description: string;
  completed: boolean;
  createdAt: number;
}
 
// Todo 타입에서 description 속성을 제거한 유틸리티 타입
type TodoPreview = Omit<Todo, "description">;
 
const todo: TodoPreview = {
  title: "Clean room",
  completed: false,
  createdAt: 1615544252770,
};
 
// Todo 타입에서 completed, createdAt 속성을 제거한 유틸리티 타입
type TodoInfo = Omit<Todo, "completed" | "createdAt">;
 
const todoInfo: TodoInfo = {
  title: "Pick up kids",
  description: "Kindergarten closes at 5pm",
};
복사

type T0 = Exclude<"a" | "b" | "c", "a">;
// type TO = "b" | "c"
 
type T1 = Exclude<"a" | "b" | "c", "a" | "b">;
// type T1 = "c"
 
type T2 = Exclude<string | number | (() => void), Function>;
// type T2 = string | number
 
type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "square"; x: number }
  | { kind: "triangle"; x: number; y: number };
 
type T3 = Exclude<Shape, { kind: "circle" }>
// type T3 = { kind: "square"; x: number; } | 
//           { kind: "triangle"; x: number; y: number; }
복사

interface Todo {
  title: string;
  description: string;
}
 
// fieldsToUpdate 인자는 Todo 타입 속성 중 일부를 가진 객체라면 모두 만족
function updateTodo(todo: Todo, fieldsToUpdate: Partial<Todo>) {
  return { ...todo, ...fieldsToUpdate };
}
 
const todo1 = {
  title: "organize desk",
  description: "clear clutter",
};
 
const todo2 = updateTodo(todo1, {
  description: "throw out trash",
});
복사

interface Props {
  a?: number;
  b?: string;
}
 
const obj: Props = { a: 5 };
 
const obj2: Required<Props> = { a: 5 };
// 🚫 Property 'b' is missing in type '{ a: number; }' but required in type 'Required<Props>'.
복사

interface Todo {
  title: string;
}
 
const todo: Readonly<Todo> = {
  title: "Delete inactive users",
};
 
todo.title = "Hello";
// 🚫 Cannot assign to 'title' because it is a read-only property.
복사

function freeze<Type>(obj: Type): Readonly<Type>;
복사

type T0 = NonNullable<string | number | undefined>;   
// type T0 = string | number
 
type T1 = NonNullable<string[] | null | undefined>;     
// type T1 = string[]
복사

declare function f1(arg: { a: number; b: string }): void;
 
type T0 = Parameters<() => string>;
// type T0 = []
 
type T1 = Parameters<(s: string) => void>;
// type T1 = [s: string]
 
type T2 = Parameters<<T>(arg: T) => T>;
// type T2 = [arg: unknown]
 
type T3 = Parameters<typeof f1>;
// type T3 = [arg: {
//     a: number;
//     b: string;
// }]
 
type T4 = Parameters<any>;
// type T4 = unknown[]
 
type T5 = Parameters<never>;
// type T5 = never
 
type T6 = Parameters<string>;
// 🚫 Type 'string' does not satisfy the constraint '(...args: any) => any'.
// 🚫 Parameters<Type>의 Type은 함수형이어야 함
// type T6 = never
 
type T7 = Parameters<Function>;
// 🚫 Type 'Function' does not satisfy the constraint '(...args: any) => any'.
//    Type 'Function' provides no match for the signature '(...args: any): any'.
// 🚫 Function 타입은 parameters에 대한 signature가 없어 불가능
// type T7 = never
복사

declare function f1(): { a: number; b: string };
 
type T0 = ReturnType<() => string>;
// type T0 = string
 
type T1 = ReturnType<(s: string) => void>;  
// type T1 = void
 
type T2 = ReturnType<<T>() => T>;  
// type T2 = unknown
 
type T3 = ReturnType<<T extends U, U extends number[]>() => T>;     
// type T3 = number[]
 
type T4 = ReturnType<typeof f1>;  
// type T4 = {
//     a: number;
//     b: string;
// }
 
type T5 = ReturnType<any>;     
// type T5 = any
 
type T6 = ReturnType<never>;     
// type T6 = never
 
type T7 = ReturnType<string>;
// 🚫 Type 'string' does not satisfy the constraint '(...args: any) => any'.
// type T7 = any
 
type T8 = ReturnType<Function>;
// 🚫 Type 'Function' does not satisfy the constraint '(...args: any) => any'.
//    Type 'Function' provides no match for the signature '(...args: any): any'.     
// type T8 = any
복사

class C {
  x = 0;
  y = 0;
}
 
type T0 = InstanceType<typeof C>;
// typeof C: C class의 constructor function
// type T0 = C
 
type T1 = InstanceType<any>;     
// type T1 = any
 
type T2 = InstanceType<never>;     
// type T2 = never
 
type T3 = InstanceType<string>;
// 🚫 Type 'string' does not satisfy the constraint 'abstract new (...args: any) => any'.
// type T3 = any
 
type T4 = InstanceType<Function>;
// 🚫 Type 'Function' does not satisfy the constraint 'abstract new (...args: any) => any'.
//    Type 'Function' provides no match for the signature 'new (...args: any): any'.
// type T4 = any
복사

function toHex(this: Number) {
  return this.toString(16);
}
 
console.log(toHex.apply(1234)); // "4d2"
 
function numberToString(n: ThisParameterType<typeof toHex>) {
  return toHex.apply(n);
}
 
console.log(numberToString(1234)); // "4d2"
복사

function toHex(this: Number) {
  return this.toString(16);
}
 
const fiveToHex: OmitThisParameter<typeof toHex> = toHex.bind(5);
 
console.log(fiveToHex()); // 5
복사