TypeScript Union and Intersection Types

let value: string | number; # can be string OR number
value = 'hello';
value = 42;

type Status = 'pending' | 'approved' | 'rejected'; # literal union
let status: Status = 'pending';

function process(value: string | number) {
    if (typeof value === 'string') { # type guard
        console.log(value.toUpperCase());
    } else {
        console.log(value.toFixed(2));
    }
}

interface Person {
    name: string;
}
interface Employee {
    employeeId: number;
}
type Staff = Person & Employee; # combine types
let staff: Staff = {
    name: 'John',
    employeeId: 123
};