Lesson 24 +10 XP

Class Interfaces (implements)

Classes can enforce structural compliance with one or more interfaces using the implements clause.

Implementing Interfaces

interface Printable {
  print(): void;
}

interface Serializable {
  serialize(): string;
}

class Invoice implements Printable, Serializable {
  constructor(public id: string, public amount: number) {}

  print(): void {
    console.log(`Invoice #${this.id}: ${this.amount}`);
  }

  serialize(): string {
    return JSON.stringify({ id: this.id, amount: this.amount });
  }
}

Interfaces vs Abstract Classes

Feature	Interface	Abstract Class
JS Code Output	Transpiles to zero JS code	Generates JS class output
Multiple Inheritance	Classes can implement multiple interfaces	Classes can inherit from only ONE base class
Implementation Code	Pure contracts (no method bodies)	Can contain method implementations & state

TL;DR

Use implements to guarantee that a class conforms to interface specifications.
A class can implement multiple interfaces separated by commas.
Interfaces leave zero footprint in the compiled JavaScript output.