PRDV402: Introduction to JavaScript II

Objects provide an interface to other code that wants to use them but maintain their own internal state. The object's internal state is kept private, meaning that it can only be accessed by the object's own methods, not from other objects. Keeping an object's internal state private, and generally making a clear division between its public interface and its private internal state, is called encapsulation.

This is a useful feature because it enables the programmer to change the internal implementation of an object without having to find and update all the code that uses it: it creates a kind of firewall between this object and the rest of the system.

For example, suppose students are allowed to study archery if they are in the second year or above. We could implement this just by exposing the student's year property, and other code could examine that to decide whether the student can take the course:

if (student.year > 1) {
  // allow the student into the class
}

The problem is, if we decide to change the criteria for allowing students to study archery - for example by also requiring the parent or guardian to give their permission - we'd need to update every place in our system that performs this test. It would be better to have a canStudyArchery() method on Student objects, that implements the logic in one place:

class Student : extends Person
    properties
       year
    constructor
       Student(name, year)
    methods
       introduceSelf()
       canStudyArchery() { return this.year > 1 }

if (student.canStudyArchery()) {
  // allow the student into the class
}

That way, if we want to change the rules about studying archery, we only have to update the Student class, and all the code using it will still work.

In many OOP languages, we can prevent other code from accessing an object's internal state by marking some properties as private. This will generate an error if code outside the object tries to access them:

class Student : extends Person
    properties
       private year
    constructor
        Student(name, year)
    methods
       introduceSelf()
       canStudyArchery() { return this.year > 1 }

student = new Student('Weber', 1)
student.year // error: 'year' is a private property of Student

In languages that don't enforce access like this, programmers use naming conventions, such as starting the name with an underscore, to indicate that the property should be considered private.