31. Abstract classes

An `abstract class` may declare `abstract` methods with no implementation, forcing subclasses to provide one. It cannot be instantiated with `new` directly.

ts
abstract class Token {
  constructor(public symbol: string) {}
  abstract transfer(to: string, amount: number): boolean;
  describe(): string {
    return `Token: ${this.symbol}`;
  }
}

class Erc20 extends Token {
  transfer(to: string, amount: number): boolean {
    console.log(`sending ${amount} to ${to}`);
    return true;
  }
}

// new Token("X"); // error: cannot instantiate an abstract class
const t = new Erc20("USDC");

Abstract classes combine shared implementation (`describe`) with an enforced contract (`transfer`) — a middle ground between a plain interface (no shared code) and a concrete base class (nothing enforced).

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