Method overriding in Javascript - Time & Space Complexity

Time Complexity: Method overriding

O(1)

Understanding Time Complexity

We want to understand how the time it takes to run code changes when we use method overriding in JavaScript.

Specifically, how does calling an overridden method affect the work done as the program runs?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

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

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

const pet = new Dog();
pet.speak();
    

This code shows a class Dog overriding the speak method from its parent Animal class, then calling the overridden method.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: A single method call to speak().
• How many times: The method is called once in this example.

How Execution Grows With Input

Since the method call happens once, the work done stays the same no matter how many objects or classes exist.

Input Size (n)	Approx. Operations
1	1 method call
10	10 method calls if called on 10 objects
100	100 method calls if called on 100 objects

Pattern observation: The time grows directly with how many times the method is called, not with the complexity of overriding itself.

Final Time Complexity

Time Complexity: O(1)

This means calling an overridden method takes a fixed amount of time each call, regardless of class hierarchy size.

Common Mistake

[X] Wrong: "Overriding a method makes the program slower as the class hierarchy grows."

[OK] Correct: The method call itself runs in constant time; the class hierarchy size does not add extra steps during the call.

Interview Connect

Understanding that method overriding does not add hidden time costs helps you explain how object-oriented features work efficiently in real code.

Self-Check

"What if the overridden method calls the parent method inside it? How would the time complexity change?"

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)