Visitor

Visitor pattern is a behavioral design pattern that allows you to separate algorithms from the objects on which they operate.

It is a way to add new operations to objects without modifying them.

Example

interface Visitor {
  visitConcreteElementA(element: ConcreteElementA): void;
  visitConcreteElementB(element: ConcreteElementB): void;
}

interface Element {
  accept(visitor: Visitor): void;
}

class ConcreteElementA implements Element {
  accept(visitor: Visitor): void {
    visitor.visitConcreteElementA(this);
  }

  operationA(): void {
    console.log('ConcreteElementA.operationA');
  }
}

class ConcreteElementB implements Element {
  accept(visitor: Visitor): void {
    visitor.visitConcreteElementB(this);
  }

  operationB(): void {
    console.log('ConcreteElementB.operationB');
  }
}

class ConcreteVisitor1 implements Visitor {
  visitConcreteElementA(element: ConcreteElementA): void {
    console.log('ConcreteVisitor1.visitConcreteElementA');
    element.operationA();
  }

  visitConcreteElementB(element: ConcreteElementB): void {
    console.log('ConcreteVisitor1.visitConcreteElementB');
    element.operationB();
  }
}

class ConcreteVisitor2 implements Visitor {
  visitConcreteElementA(element: ConcreteElementA): void {
    console.log('ConcreteVisitor2.visitConcreteElementA');
    element.operationA();
  }

  visitConcreteElementB(element: ConcreteElementB): void {
    console.log('ConcreteVisitor2.visitConcreteElementB');
    element.operationB();
  }
}

const elements: Element[] = [new ConcreteElementA(), new ConcreteElementB()];

const visitor1 = new ConcreteVisitor1();
const visitor2 = new ConcreteVisitor2();

for (const element of elements) {
  element.accept(visitor1);
}

for (const element of elements) {
  element.accept(visitor2);
}

Output

ConcreteVisitor1.visitConcreteElementA
ConcreteElementA.operationA
ConcreteVisitor1.visitConcreteElementB
ConcreteElementB.operationB
ConcreteVisitor2.visitConcreteElementA
ConcreteElementA.operationA
ConcreteVisitor2.visitConcreteElementB
ConcreteElementB.operationB

Visitor

Example

Output

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