====== Utility Types ====== ===== Partial ===== Constructs a type with all properties of T set to optional. This utility will return a type that represents all subsets of a given type. Example


interface Todo {
    title: string;
    description: string;
}

function updateTodo(todo: Todo, fieldsToUpdate: Partial) {
    return { ...todo, ...fieldsToUpdate };
}

const todo1 = {
    title: 'organize desk',
    description: 'clear clutter',
};

const todo2 = updateTodo(todo1, {
    description: 'throw out trash',
});

===== Readonly ===== Constructs a type with all properties of T set to readonly, meaning the properties of the constructed type cannot be reassigned. Example #


interface Todo {
    title: string;
}

const todo: Readonly = {
    title: 'Delete inactive users',
};

todo.title = 'Hello'; // Error: cannot reassign a readonly property

This utility is useful for representing assignment expressions that will fail at runtime (i.e. when attempting to reassign properties of a frozen object). ===== Object.freeze ===== function freeze(obj: T): Readonly; Record # Constructs a type with a set of properties K of type T. This utility can be used to map the properties of a type to another type. Example #


interface PageInfo {
    title: string;
}

type Page = 'home' | 'about' | 'contact';

const x: Record = {
    about: { title: 'about' },
    contact: { title: 'contact' },
    home: { title: 'home' },
};

===== Pick ===== Constructs a type by picking the set of properties K from T. Example #


interface Todo {
    title: string;
    description: string;
    completed: boolean;
}

type TodoPreview = Pick;

const todo: TodoPreview = {
    title: 'Clean room',
    completed: false,
};

===== Omit ===== Constructs a type by picking all properties from T and then removing K. Example #


interface Todo {
    title: string;
    description: string;
    completed: boolean;
}

type TodoPreview = Omit;

const todo: TodoPreview = {
    title: 'Clean room',
    completed: false,
};

===== Exclude ===== Constructs a type by excluding from T all properties that are assignable to U. Example #


type T0 = Exclude<"a" | "b" | "c", "a">;  // "b" | "c"
type T1 = Exclude<"a" | "b" | "c", "a" | "b">;  // "c"
type T2 = Exclude void), Function>;  // string | number

===== Extract ===== Constructs a type by extracting from T all properties that are assignable to U. Example #


type T0 = Extract<"a" | "b" | "c", "a" | "f">;  // "a"
type T1 = Extract void), Function>;  // () => void

===== NonNullable ===== Constructs a type by excluding null and undefined from T. Example #


type T0 = NonNullable;  // string | number
type T1 = NonNullable;  // string[]

===== ReturnType ===== Constructs a type consisting of the return type of function T. Example #


type T0 = ReturnType<() => string>;  // string
type T1 = ReturnType<(s: string) => void>;  // void
type T2 = ReturnType<(() => T)>;  // {}
type T3 = ReturnType<(() => T)>;  // number[]
type T4 = ReturnType;  // { a: number, b: string }
type T5 = ReturnType;  // any
type T6 = ReturnType;  // any
type T7 = ReturnType;  // Error
type T8 = ReturnType;  // Error

===== InstanceType ===== Constructs a type consisting of the instance type of a constructor function type T. Example #


class C {
    x = 0;
    y = 0;
}

type T0 = InstanceType;  // C
type T1 = InstanceType;  // any
type T2 = InstanceType;  // any
type T3 = InstanceType;  // Error
type T4 = InstanceType;  // Error

===== Required ===== Constructs a type consisting of all properties of T set to required. Example #


interface Props {
    a?: number;
    b?: string;
};

const obj: Props = { a: 5 }; // OK

const obj2: Required = { a: 5 }; // Error: property 'b' missing

===== ThisType ===== This utility does not return a transformed type. Instead, it serves a marker for a contextual this type. Note that the --noImplicitThis flag must be enabled to use this utility. Example #


// Compile with --noImplicitThis

type ObjectDescriptor = {
    data?: D;
    methods?: M & ThisType;  // Type of 'this' in methods is D & M
}

function makeObject(desc: ObjectDescriptor): D & M {
    let data: object = desc.data || {};
    let methods: object = desc.methods || {};
    return { ...data, ...methods } as D & M;
}

let obj = makeObject({
    data: { x: 0, y: 0 },
    methods: {
        moveBy(dx: number, dy: number) {
            this.x += dx;  // Strongly typed this
            this.y += dy;  // Strongly typed this
        }
    }
});

obj.x = 10;
obj.y = 20;
obj.moveBy(5, 5);

위의 예제에서 makeObject 인수의 methods 객체는 thisType 을 포함하는 컨텍스트 유형을 가지므로 메서드 객체 내의 메소드에있는이 유형은 {x : number, y : number} 및 {moveBy (dx : number, dy : number) : number}. 메소드 프로퍼티의 타입이 추론 타겟인지, 메소드의이 타입에 대한 소스인지를 주목하십시오. ThisType 마커 인터페이스는 lib.d.ts에 선언 된 빈 인터페이스입니다. 인터페이스는 객체 리터럴의 컨텍스트 유형에서 인식되는 것 이외에 빈 인터페이스처럼 작동합니다.