Visitor in JavaScript

The visitor design pattern concerns itself with being able to add functionality to objects without modifying the structure of them.

With classical inheritance, we often end up with a “base class” that is not used directly; it’s used as an “abstract class,” from which “concrete” classes inherit from our “base class.” For example, with BankAccount and BankAccountWithInterest, our class diagram would look as follows, where BankAccountWithInterest extends BankAccount and overrides setBalance.

Figure 3.2: A class diagram for BankAccountWithInterest inheriting from BankAccount

What we can do with the visitor pattern is define BankAccount, which accepts a visitor and an InterestRateVisitor visitor class. As a class diagram, it looks as follows. BankAccount and InterestRateVisitor are not linked via inheritance; they will be linked at runtime when InterestRateVisitor is called...