TypeScript - Data Modifiers

In object-oriented programming, the concept of 'Encapsulation' is used to make class members public or private i.e. a class can control the visibility of its data members. This is done using access modifiers.

There are three types of access modifiers in TypeScript: public, private and protected.

public

By default, all members of a class in TypeScript are public. All the public members can be accessed anywhere without any restrictions.

Example: public
class Employee {
    public empCode: string;
    empName: string;
}

let emp = new Employee();
emp.empCode = 123;
emp.empName = "Swati";

In the above example, empCode and empName are declared as public. So, they can be accessible outside of the class using an object of the class.

Please notice that there is not any modifier applied before empName, as TypeScript treats properties and methods as public by default if no modifier is applied to them.

private

The private access modifier ensures that class members are visible only to that class and are not accessible outside the containing class.

Example: private
class Employee {
    private empCode: number;
    empName: string;
}

let emp = new Employee();
emp.empCode = 123; // Compiler Error
emp.empName = "Swati";//OK

In the above example, we have marked the member empCode as private. Hence, when we create an object emp and try to access the emp.empCode member, it will give an error.

protected

The protected access modifier is similar to the private access modifier, except that protected members can be accessed using their deriving classes.

Example: protected
class Employee {
    public empName: string;
    protected empCode: number;

    constructor(name: string, code: number){
        this.empName = name;
        this.empCode = code;
    }
}

class SalesEmployee extends Employee{
    private department: string;
    
    constructor(name: string, code: number, department: string) {
        super(name, code);
        this.department = department;
    }
}

let emp = new SalesEmployee("John Smith", 123, "Sales");
empObj.empCode; //Compiler Error

In the above example, we have a class Employee with two members, public empName and protected property empCode. We create a subclass SalesEmployee that extends from the parent class Employee. If we try to access the protected member from outside the class, as emp.empCode, we get the following compilation error:

error TS2445: Property 'empCode' is protected and only accessible within class 'Employee' and its subclasses.

In addition to the access modifiers, TypeScript provides two more keywords: readOnly and static. Learn about them next.