Interfaces vs type aliases, extends vs implements

Model answer. Both describe the shape of objects, and for most object types they are interchangeable.

• `interface` describes object and class shapes; it can be reopened (declaration merging) and extended with extends.
• `type` can name any type: unions ('asc' | 'desc'), tuples, primitives, mapped and conditional types. It cannot be reopened.

interface Person { name: string }
interface Employee extends Person { empId: number }        // interface inheritance

type Status = 'active' | 'blocked';                         // only possible with type
type Admin = Employee & { permissions: string[] };          // intersection

interface Payable { pay(amount: number): void }
class Contractor implements Payable {                       // class promises to match
  pay(amount: number) { console.log(`paid ${amount}`); }
}

• `extends`: one interface (or class) inherits members from another.
• `implements`: a class promises to satisfy an interface; it inherits nothing, and the compiler only checks the class.

A common team convention: interface for object shapes and public APIs, type for unions and compositions.