What is the difference between implements and extends in TypeScript?

extends — inherits from a parent class or interface. Gets all properties, methods, and constructor logic.

implements — declares that a class satisfies an interface contract, but does NOT inherit implementation.

// Interface contract
interface Serializable {
  serialize(): string;
}

interface Loggable {
  log(): void;
}

// extends — inherits implementation
class Animal {
  constructor(public name: string) {}
  breathe() { return 'breathing'; }
}

class Dog extends Animal {
  // Inherits name, breathe() from Animal
  bark() { return 'woof'; }
}

// implements — satisfy contract without inheriting
class User implements Serializable, Loggable {
  constructor(public name: string, public email: string) {}

  // MUST implement all interface methods
  serialize() { return JSON.stringify({ name: this.name, email: this.email }); }
  log() { console.log(this.name); }
}

// Combining both
class AdminUser extends User implements Comparable<AdminUser> {
  constructor(name: string, email: string, public adminLevel: number) {
    super(name, email); // must call parent constructor
  }

  compareTo(other: AdminUser): number {
    return this.adminLevel - other.adminLevel;
  }
}

Interface implements itself (structural check):

// A class can implement an interface to get compile-time checking
// that it fulfills the contract — even without runtime cost
class Repository implements IRepository<User> {
  // TypeScript errors if any method is missing
}