Visitor Design Pattern

Structural: Visitor Pattern

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Can you explain the visitor design pattern?

View Answer:

Interview Response: Visitor is a behavioral design pattern that lets you detach algorithms from the objects on which they act. The Visitor pattern adds new methods to a group of objects without affecting them, and the new logic gets housed in a distinct entity known as the Visitor.

Code Example:

The objects participating in this pattern are:

ObjectStructure -- example code: employees array

• maintains a collection of elements that can get iterated over

Elements -- example code: Employee objects

• defines an accept method that accepts visitor objects
• in the accept method, the Visitor's visit method gets invoked with 'this' as a parameter

Visitor -- example code: ExtraSalary, ExtraVacation

• implements a visit method. The element getting visited is the argument when the element's changes get made.

let Employee = function (name, salary, vacation) {
  let self = this;

  this.accept = function (visitor) {
    visitor.visit(self);
  };

  this.getName = function () {
    return name;
  };

  this.getSalary = function () {
    return salary;
  };

  this.setSalary = function (sal) {
    salary = sal;
  };

  this.getVacation = function () {
    return vacation;
  };

  this.setVacation = function (vac) {
    vacation = vac;
  };
};

let ExtraSalary = function () {
  this.visit = function (emp) {
    emp.setSalary(emp.getSalary() * 1.1);
  };
};

let ExtraVacation = function () {
  this.visit = function (emp) {
    emp.setVacation(emp.getVacation() + 2);
  };
};

function run() {
  let employees = [
    new Employee('John', 10000, 10),
    new Employee('Mary', 20000, 21),
    new Employee('Boss', 250000, 51),
  ];

  let visitorSalary = new ExtraSalary();
  let visitorVacation = new ExtraVacation();

  for (let i = 0, len = employees.length; i < len; i++) {
    let emp = employees[i];

    emp.accept(visitorSalary);
    emp.accept(visitorVacation);
    console.log(
      emp.getName() +
        ': $' +
        emp.getSalary() +
        ' and ' +
        emp.getVacation() +
        ' vacation days'
    );
  }
}

run();

/*

OUTPUT:

John: $11000 and 12 vacation days
Mary: $22000 and 23 vacation days
Boss: $275000 and 53 vacation days

*/

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To which pattern family does the Visitor pattern belong?

View Answer:

Interview Response: The Visitor pattern is part of the Behavioral design pattern set.

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When should you utilize the JavaScript Visitor Pattern?

View Answer:

Interview Response: We can use the visitor pattern when:

• Similar procedures must get done on various data structure objects.
• Specific operations must get carried out on multiple items in the data structure.
• You wish to make libraries or frameworks more extensible.

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What are some of the benefits of using the Visitor pattern?

View Answer:

Interview Response: Benefits of the Visitor Pattern

• The principle of open/closed. You may add new behavior that works with objects of various classes without modifying the classes themselves.
• Single Responsibility Principle. You can move multiple versions of the same behavior into the same class.
• While working with various objects, a visitor object might get helpful information. This information is helpful if you wish to traverse a complicated object structure, such as an object tree, and apply the Visitor to each item in the structure.

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What are some of the Visitor pattern's drawbacks?

View Answer:

Interview Response: Drawbacks of the Visitor Pattern.

• Every time a class is added or withdrawn from the element hierarchy, you must notify all visitors.
• Visitors may not have access to the private fields and methods of the components they get expected to operate.

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