Advanced types

Union Type

• To allow a property to have multiple types
• Use the | symbol to separate types

function formatCommandline(command: string[] | string) {
    let line = '';
    if (typeof command === 'string') {
        line = command.trim();
    } else {
        line = command.join(' ').trim();
    }
}

Tips

• Union Types are most commonly used to safely represent potential error cases

Intersection Type

• An intersection type combines multiple types into one

export interface ProgrammerEmployee {
  firstName: string;
  lastName: string;
  favoriteLanguage: string;
}

export interface BillableResource {
  hourlyRate: number;
}

let billableProgrammer: ProgrammerEmployee & BillableResource = {
  firstName: "John",
  lastName: "Doe",
  favoriteLanguage: "TypeScript",
  hourlyRate: 100,
};

console.log(billableProgrammer);