Inheritance

Inheritance lets you create type hierarchies and reuse code. A parent class and its subclasses share state and behavior.

JavaScript now supports inheritance with syntax similar to Java/C++.

class Student {
  constructor(name, email) {
    this.name = name;
    this.email = email;
  }
}

class GradStudent extends Student {
  constructor(name, email, advisor) {
    super(name, email);
    this.advisor = advisor;
  }
}

const john = new Student("John Doe", "john@email.com");
const jane = new GradStudent("Jane Doe", "jane@email.com", "Prof. Smith");

console.log(john instanceof Student); // true
console.log(jane instanceof Student); // true
console.log(john instanceof GradStudent); // false
console.log(jane instanceof GradStudent); // true

console.log(john); // {"name":"John Doe","email":"john@email.com"}
console.log(jane); // {"name":"Jane Doe","email":"jane@email.com","advisor":"Prof. Smith"}

• The extends keyword declares GradStudent as a subclass of Student.
• The super keyword calls the parent class’s constructor.
• The super() call in the GradStudent constructor lets the parent class’s constructor set name and email.
• If you omit the constructor in a subclass, JavaScript provides a default one that simply calls super(...args). This differs from Java or C++, where you must explicitly define constructors.

Custom Error Objects

A good example of inheritance in JavaScript is creating custom error objects. JavaScript provides built-in error objects like Error, SyntaxError, ReferenceError, etc. You can also create custom error objects by extending the built-in Error class:

class CustomError extends Error {
  constructor(message) {
    super(message);
    this.name = "CustomError";
  }
}

function riskyOperation() {
  throw new CustomError("Something went wrong in the risky operation");
}

try {
  riskyOperation();
} catch (error) {
  if (error instanceof CustomError) {
    console.error("A custom error occurred:", error.message);
  } else {
    console.error("An unexpected error occurred:", error);
  }
}