13. Interfaces implemented by classes

A class can `implements` an interface, promising to provide all its members. This is checked at compile time only — `implements` adds no runtime behaviour.

ts
interface Transferable {
  transfer(to: string, amount: number): boolean;
}

class Token implements Transferable {
  balances: Record<string, number> = {};
  transfer(to: string, amount: number): boolean {
    this.balances[to] = (this.balances[to] ?? 0) + amount;
    return true;
  }
}

A class can implement multiple interfaces (`class X implements A, B`), and interfaces can extend other interfaces, letting you compose small, focused contracts (e.g. `Mintable`, `Burnable`, `Transferable`) into a richer one.

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