Understanding protected

Understanding protected

The protected modifier acts much like the private modifier with the exception that members declared protected can also be accessed by instances of deriving classes. For example,

class Person {
  protected name: string;
  constructor(name: string) { this.name = name; }
}
 
class Employee extends Person {
  private department: string;
 
  constructor(name: string, department: string) {
    super(name);
    this.department = department;
  }
 
  public getElevatorPitch() {
    return `Hello, my name is ${this.name} and I work in ${this.department}.`;
  }
}
 
let howard = new Employee("Howard", "Sales");
console.log(howard.getElevatorPitch());
console.log(howard.name); // error

Notice that while we can’t use name from outside of Person, we can still use it from within an instance method of Employee because Employee derives from Person.

A constructor may also be marked protected. This means that the class cannot be instantiated outside of its containing class, but can be extended. For example,

class Person {
  protected name: string;
  protected constructor(theName: string) { this.name = theName; }
}
 
// Employee can extend Person
class Employee extends Person {
  private department: string;
 
  constructor(name: string, department: string) {
    super(name);
    this.department = department;
  }
 
  public getElevatorPitch() {
    return `Hello, my name is ${this.name} and I work in ${this.department}.`;
  }
}
 
let howard = new Employee("Howard", "Sales");
let john = new Person("John"); // Error: The 'Person' constructor is protected

Links:

https://www.typescriptlang.org/docs/handbook/classes.html#understanding-protected

doc_TypeScript

2025-01-10 15:47:30

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