타입 가드

typeof [객체] === '[타입]'

[메소드 이름 or 프로퍼티 이름] in [객체]

[클래스] instanceof [메소드 이름 or 프로퍼티 이름]

typeof, in, instanceof 타입 가드를 사용할 수 있다.

인터페이스는 자바스크립트로 컴파일되지 않으니 instanceof 를 사용할 수 없다.

type Admin = {
    name: string;
    privileges: string[];

};

type Employee = {
    name: string;
    startDate: Date;
};

// interface ElevatedEmployee extends Admin, Employee {}

type ElevatedEmployee = Admin & Employee;

const e1: ElevatedEmployee = {
    name: 'Max',
    privileges: ['create-server'],
    startDate: new Date()
}

type Combinable = string | number;
type Numeric = number | boolean;

type Universal = Combinable & Numeric;

function add2(a: Combinable, b: Combinable) {
    if (typeof a === 'string' || typeof b === 'string') {
        return a.toString() + b.toString();
    }
    return a + b;
}

type UnknownEmployee = Employee | Admin;

function printEmployeeInformation(emp: UnknownEmployee) {
    console.log('Name: ' + emp.name);
    if ('privileges' in emp) {
        console.log('Privileges: ' + emp.privileges);
    }
    if ('startDate' in emp) {
        console.log('StartDate: ' + emp.startDate);
    }
}

printEmployeeInformation({ name: 'a', startDate: new Date() });

class Car {
    drive() {
        console.log('Driving...');
    }
}

class Truck {
    drive() {
        console.log('Driving a truck...');
    }

    loadCargo(amount: number) {
        console.log('Loading cargo ... ' + amount);
    }
}

type Vehicle = Car | Truck;

const v1 = new Car();
const v2 = new Truck();

function useVehicle(vehicle: Vehicle) {
    vehicle.drive();
    if (vehicle instanceof Truck) {
        vehicle.loadCargo(1000);
    }
}

useVehicle(v1);
useVehicle(v2);