Visitor

Defines a new operation to a class without change.

• The Visitor pattern defines a new operation to a collection of objects without changing the objects themselves.

• The new logic resides in a separate object called the Visitor.

• Visitors are useful when building extensibility in a library or framework.

• In Visitor pattern, we use a visitor class which changes the executing algorithm of an element class.

• By this way, execution algorithm of element can vary as and when visitor varies.

EX:
var Employee = function (name, salary, vacation) {
    var 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;
    };
};
 
var ExtraSalary = function () {
    this.visit = function (emp) {
        emp.setSalary(emp.getSalary() * 1.1);
    };
};
 
var ExtraVacation = function () {
    this.visit = function (emp) {
        emp.setVacation(emp.getVacation() + 2);
    };
};

function run() {        
    var employees = [
        new Employee("John", 10000, 10),
        new Employee("Mary", 20000, 21),
        new Employee("Boss", 250000, 51)
    ]; 
    var visitorSalary = new ExtraSalary();
    var visitorVacation = new ExtraVacation();        
    for (var i = 0, len = employees.length; i < len; i++) {
        var emp = employees[i];            
        emp.accept(visitorSalary);
        emp.accept(visitorVacation);
        console.log(emp.getName() + ": $" + emp.getSalary() +
            " and " + emp.getVacation() + " vacation days");
    }
}

run();