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Mastering TypeScript

Protected and Abstract Classes

Explore how to use protected and abstract classes in TypeScript to control access to properties and methods within class hierarchies. Understand how protected members allow derived classes to access certain properties while keeping them hidden outside. Discover how abstract classes serve as blueprints for other classes, enabling shared properties and methods in your TypeScript projects.

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Protected classes

Classes can mark both properties and functions with the protected keyword. If a property is marked as protected, then it is not accessible outside of the class itself, similar to the behavior of the private keyword.

It is, however, accessible to derived classes, which is different from private variables that are not accessible to derived classes, which can be seen in the following example:

TypeScript 4.9.5
// Define the BaseClassProtected class
class BaseClassProtected {
  // Declare a protected property "id" of type number
  protected id: number;
  // Declare a private property "name" of type string with a default value of ""
  private name: string = "";
  // Constructor which takes a number argument "id"
  constructor(id: number) {
    // Set the value of the "id" property to the argument
    this.id = id;
  }
}
// Define the AccessProtected class which extends the BaseClassProtected class
class AccessProtected
  extends BaseClassProtected {
  // Constructor which takes a number argument "id"
  constructor(id: number) {
    // Call the constructor of the BaseClassProtected class with the argument
    super(id);
    // Log the value of the "id" property from the BaseClassProtected class
    console.log(`base.id = ${this.id}`);
    // Log the value of the "name" property from the BaseClassProtected class
    console.log(`base.name = ${this.name}`);
  }
}
Accessing protected properties

  • We define a class named BaseClassProtected from lines 2–13 that has a protected property named id of type number and a private property named name of type string. We are setting the value of the protected id property within the constructor.

  • We then define a class named AccessProtected from lines 16–27 that derives from the ...