ChainOfResponsibility.ts

/**
 * The Handler interface declares a method for building the chain of handlers.
 * It also declares a method for executing a request.
 */
interface Handler {
  setNext(handler: Handler): Handler;

  handle(request: string): string;
}

/**
 * The default chaining behavior can be implemented inside a base handler class.
 */
abstract class AbstractHandler implements Handler {
  private nextHandler: Handler;

  public setNext(handler: Handler): Handler {
    this.nextHandler = handler;
    // Returning a handler from here will let us link handlers in a
    // convenient way like this:
    // monkey.setNext(squirrel).setNext(dog);
    return handler;
  }

  public handle(request: string): string {
    if (this.nextHandler) {
      return this.nextHandler.handle(request);
    }

    return null;
  }
}

/**
 * All Concrete Handlers either handle a request or pass it to the next handler
 * in the chain.
 */
class MonkeyHandler extends AbstractHandler {
  public handle(request: string): string {
    if (request === 'Banana') {
      return `Monkey: I'll eat the ${request}.`;
    }
    return super.handle(request);
  }
}

class SquirrelHandler extends AbstractHandler {
  public handle(request: string): string {
    if (request === 'Nut') {
      return `Squirrel: I'll eat the ${request}.`;
    }
    return super.handle(request);
  }
}

class DogHandler extends AbstractHandler {
  public handle(request: string): string {
    if (request === 'MeatBall') {
      return `Dog: I'll eat the ${request}.`;
    }
    return super.handle(request);
  }
}

/**
 * The client code is usually suited to work with a single handler. In most
 * cases, it is not even aware that the handler is part of a chain.
 */
function clientCode(handler: Handler) {
  const foods = ['Nut', 'Banana', 'Cup of coffee'];

  for (const food of foods) {
    console.log(`Client: Who wants a ${food}?`);

    const result = handler.handle(food);
    if (result) {
      console.log(`  ${result}`);
    } else {
      console.log(`  ${food} was left untouched.`);
    }
  }
}

/**
 * The other part of the client code constructs the actual chain.
 */
const monkey = new MonkeyHandler();
const squirrel = new SquirrelHandler();
const dog = new DogHandler();

monkey.setNext(squirrel).setNext(dog);

/**
 * The client should be able to send a request to any handler, not just the
 * first one in the chain.
 */
console.log('Chain: Monkey > Squirrel > Dog\n');
clientCode(monkey);
console.log('');

console.log('Subchain: Squirrel > Dog\n');
clientCode(squirrel);

/** OUTPUT
Chain: Monkey > Squirrel > Dog

Client: Who wants a Nut?
  Squirrel: I'll eat the Nut.
Client: Who wants a Banana?
  Monkey: I'll eat the Banana.
Client: Who wants a Cup of coffee?
  Cup of coffee was left untouched.

Subchain: Squirrel > Dog

Client: Who wants a Nut?
  Squirrel: I'll eat the Nut.
Client: Who wants a Banana?
  Banana was left untouched.
Client: Who wants a Cup of coffee?
  Cup of coffee was left untouched.
 */