Method overriding in Javascript

Introduction

Method overriding lets a child object change how a method works from its parent. This helps customize behavior while keeping the same method name.

When you want a child object to do something different than the parent for the same action.

When you have a general method in a parent and need specific versions in children.

When you want to reuse code but change small parts in child objects.

When building games where different characters act differently but share common actions.

When creating UI components that share a base but behave uniquely.

Syntax

Javascript

class Parent {
  methodName() {
    // parent method code
  }
}

class Child extends Parent {
  methodName() {
    // child method code overrides parent
  }
}

The child class uses the same method name to replace the parent's method.

Use extends to create a child class from a parent class.

Examples

The Dog class changes the speak method to bark instead of the generic animal sound.

Javascript

class Animal {
  speak() {
    console.log('Animal speaks');
  }
}

class Dog extends Animal {
  speak() {
    console.log('Dog barks');
  }
}

The Car class overrides start to show a specific message.

Javascript

class Vehicle {
  start() {
    console.log('Vehicle starting');
  }
}

class Car extends Vehicle {
  start() {
    console.log('Car engine starting');
  }
}

Sample Program

This program shows method overriding. The Student class changes the greet method from Person. When we call greet on each, they print different messages.

Javascript

class Person {
  greet() {
    console.log('Hello from Person');
  }
}

class Student extends Person {
  greet() {
    console.log('Hello from Student');
  }
}

const person = new Person();
const student = new Student();

person.greet();
student.greet();

Important Notes

Overriding only works if the method name is exactly the same in parent and child.

You can call the parent's method inside the child using super.methodName() if needed.

Summary

Method overriding lets child classes change parent methods.

Use the same method name in child to replace parent behavior.

It helps customize shared actions for different objects.

Practice

1. What does method overriding allow a child class to do in JavaScript?

easy

A. Replace a method from the parent class with a new version

B. Create a new method with a different name

C. Delete a method from the parent class

D. Call a method without defining it

Solution

1. Step 1: Recall method overriding concept

   Method overriding means a child class uses the same method name as the parent to change its behavior.

2. Step 2: Identify what overriding does

   It replaces the parent's method with a new version in the child class.

3. Final Answer:

   Replace a method from the parent class with a new version -> Option A

4. Quick Check:

   Method overriding = Replace parent method [OK]

Hint: Same method name in child replaces parent method [OK]

Common Mistakes:

• Thinking overriding creates a new method with a different name
• Believing overriding deletes parent methods
• Confusing overriding with method calling

2. Which of the following is the correct syntax to override a method greet() in a child class Dog that extends Animal?

easy

A. class Dog extends Animal { greet = () => { console.log('Woof!'); } }

B. class Dog extends Animal { function greet() { console.log('Woof!'); } }

C. class Dog extends Animal { greet() { console.log('Woof!'); } }

D. class Dog extends Animal { var greet() { console.log('Woof!'); } }

Solution

1. Step 1: Check method syntax in class

   In JavaScript classes, methods are defined without keywords like function or var inside the class body.

2. Step 2: Identify correct method override syntax

   class Dog extends Animal { greet() { console.log('Woof!'); } } correctly defines greet() as a method inside the class body.

3. Final Answer:

   class Dog extends Animal { greet() { console.log('Woof!'); } } -> Option C

4. Quick Check:

   Method override syntax = methodName() { } [OK]

Hint: Use methodName() { } inside class to override [OK]

Common Mistakes:

• Using function keyword inside class method
• Using var or let to define methods inside class
• Using arrow functions incorrectly for overriding

3. What will be the output of the following code?

class Animal {
  speak() { return 'Animal sound'; }
}
class Cat extends Animal {
  speak() { return 'Meow'; }
}
const pet = new Cat();
console.log(pet.speak());

medium

A. 'Meow'

B. Error: speak is not a function

C. undefined

D. 'Animal sound'

Solution

1. Step 1: Identify method overriding in Cat class

   The speak() method in Cat replaces the one in Animal.

2. Step 2: Check which method is called on pet

   Since pet is an instance of Cat, Cat's speak() runs, returning 'Meow'.

3. Final Answer:

   'Meow' -> Option A

4. Quick Check:

   Child method runs = 'Meow' [OK]

Hint: Child class method overrides parent method output [OK]

Common Mistakes:

• Expecting parent method output instead of child
• Confusing instance type with method called
• Assuming undefined or error due to override

4. Find the error in this code that tries to override the run() method:

class Vehicle {
  run() {
    console.log('Vehicle running');
  }
}
class Car extends Vehicle {
  run = () => {
    console.log('Car running fast');
  }
}

medium

A. run method should be static to override

B. Arrow function syntax is invalid for method overriding in class

C. Missing super.run() call inside overridden method

D. No error, code works fine

Solution

1. Step 1: Check method definition style in class

   Class methods should be defined using method syntax, not arrow functions assigned to properties.

2. Step 2: Identify why arrow function causes error

   Using run = () => {} creates a class field, not a proper method, so it does not override the parent method correctly.

3. Final Answer:

   Arrow function syntax is invalid for method overriding in class -> Option B

4. Quick Check:

   Override needs method syntax, not arrow function [OK]

Hint: Override with method(), not arrow function [OK]

Common Mistakes:

• Using arrow functions inside class for overriding
• Forgetting method syntax rules
• Thinking super call is mandatory for override

5. You have a parent class Shape with a method area() that returns 0. You want to create a child class Rectangle that overrides area() to return width x height. Which code correctly implements this?

hard

A. class Rectangle extends Shape {
constructor(w, h) {
super();
this.w = w;
this.h = h;
}
area = () => this.w * this.h;
}

B. class Rectangle extends Shape {
constructor(w, h) {
this.w = w;
this.h = h;
}
area() {
return w * h;
}
}

C. class Rectangle extends Shape {
area(w, h) {
return w * h;
}
}

D. class Rectangle extends Shape {
constructor(w, h) {
super();
this.w = w;
this.h = h;
}
area() {
return this.w * this.h;
}
}

Solution

1. Step 1: Check constructor and property setup

   class Rectangle extends Shape { constructor(w, h) { super(); this.w = w; this.h = h; } area() { return this.w * this.h; } } correctly calls super() and assigns width and height to this.

2. Step 2: Verify method overriding and usage of properties

   The area() method uses this.w and this.h to calculate area, correctly overriding the parent method.

3. Final Answer:

   class Rectangle extends Shape { constructor(w, h) { super(); this.w = w; this.h = h; } area() { return this.w * this.h; } } -> Option D

4. Quick Check:

   Override with constructor + this properties + method [OK]

Hint: Use constructor with super() and this.prop for override [OK]

Common Mistakes:

• Forgetting to call super() in constructor
• Using parameters inside area() instead of properties
• Defining area as arrow function (less common but works differently)