Why classes are introduced in Javascript - See It in Action

Understanding Why Classes Are Introduced in JavaScript

📖 Scenario: Imagine you are building a simple game where you need to create many characters. Each character has a name and a health value. You want an easy way to create and manage these characters.

🎯 Goal: You will learn why JavaScript classes were introduced by first creating characters using simple objects, then adding a helper function, and finally using a class to make the code cleaner and easier to manage.

📋 What You'll Learn

Create an object for a character with name and health

Create a function to make new characters

Create a class to define characters

Create instances of the class

Print character details

💡 Why This Matters

🌍 Real World

Classes help organize code when building games, apps, or websites where many similar things exist, like users, products, or characters.

💼 Career

Understanding classes is important for writing clean, reusable code in JavaScript, which is a key skill for many programming jobs.

1

Create a character object

Create an object called character1 with properties name set to 'Hero' and health set to 100.

Javascript

// Create an object called character1 with name 'Hero' and health 100
// Your code here

Hint

Use curly braces {} to create an object with the properties name and health.

2

Create a function to make characters

Create a function called createCharacter that takes name and health as parameters and returns an object with those properties.

Javascript

const character1 = { name: 'Hero', health: 100 };

// Create a function called createCharacter(name, health) that returns an object with name and health
// Your code here

Hint

Use a function that returns an object with the given name and health.

3

Create a class for characters

Create a class called Character with a constructor that takes name and health and sets them as properties.

Javascript

const character1 = { name: 'Hero', health: 100 };

function createCharacter(name, health) {
    return { name: name, health: health };
}

// Create a class called Character with a constructor(name, health) that sets this.name and this.health
// Your code here

Hint

Use the class keyword and a constructor method to set properties.

4

Create instances and print details

Create two instances of Character called hero with name 'Hero' and health 100, and villain with name 'Villain' and health 80. Then print their names and health using console.log.

Javascript

const character1 = { name: 'Hero', health: 100 };

function createCharacter(name, health) {
    return { name: name, health: health };
}

class Character {
    constructor(name, health) {
        this.name = name;
        this.health = health;
    }
}

// Create hero and villain instances and print their details
// Your code here

Hint

Use new Character(...) to create instances and console.log with template strings to print.

Practice

1. Why were classes introduced in JavaScript?

easy

A. To remove the need for variables

B. To replace all functions with a new syntax

C. To make JavaScript run faster

D. To group data and related actions together in one place

Solution

1. Step 1: Understand the purpose of classes

   Classes help organize data and functions that belong together, making code easier to manage.

2. Step 2: Compare options with class purpose

   Only grouping data and actions matches the reason classes were introduced.

3. Final Answer:

   To group data and related actions together in one place -> Option D

4. Quick Check:

   Classes group data and actions = D [OK]

Hint: Classes bundle data and actions together [OK]

Common Mistakes:

• Thinking classes make code run faster
• Believing classes remove variables
• Confusing classes with replacing all functions

2. Which of the following is the correct way to define a class in JavaScript?

easy

A. var Car = () => { }

B. function Car() { }

C. class Car { constructor() { } }

D. class Car() { }

Solution

1. Step 1: Recall class syntax in JavaScript

   The correct syntax uses the keyword class followed by the class name and curly braces.

2. Step 2: Check each option

   class Car { constructor() { } } uses correct syntax with class Car { constructor() { } }. class Car() { } has wrong parentheses after class name.

3. Final Answer:

   class Car { constructor() { } } -> Option C

4. Quick Check:

   Correct class syntax = A [OK]

Hint: Class syntax uses 'class Name { constructor() {} }' [OK]

Common Mistakes:

• Adding parentheses after class name
• Using function syntax instead of class
• Confusing arrow functions with classes

3. What will be the output of this code?

class Dog {
  constructor(name) {
    this.name = name;
  }
  bark() {
    return `${this.name} says woof!`;
  }
}
const dog1 = new Dog('Buddy');
console.log(dog1.bark());

medium

A. "Buddy says woof!"

B. "dog1 says woof!"

C. undefined says woof!

D. Error: bark is not a function

Solution

1. Step 1: Understand class and method usage

   The class Dog sets the name property via constructor and bark() returns a string using that name.

2. Step 2: Trace the code execution

   dog1 is created with name 'Buddy'. Calling dog1.bark() returns 'Buddy says woof!'.

3. Final Answer:

   "Buddy says woof!" -> Option A

4. Quick Check:

   dog1.bark() = "Buddy says woof!" [OK]

Hint: Constructor sets name; bark() uses it in string [OK]

Common Mistakes:

• Expecting dog1 as name instead of 'Buddy'
• Thinking bark() is undefined
• Confusing method call syntax

4. Find the error in this class definition:

class Person {
  constructor(name) {
    this.name = name;
  }
  greet() {
    console.log('Hello, ' + name);
  }
}
const p = new Person('Alice');
p.greet();

medium

A. Missing 'this' before 'name' inside greet() method

B. Constructor is missing parentheses

C. Class name should be lowercase

D. greet() method should return a string, not console.log

Solution

1. Step 1: Check variable usage inside greet()

   The method uses name without this., so it looks for a local variable which does not exist.

2. Step 2: Identify correct property access

   It should use this.name to access the instance property.

3. Final Answer:

   Missing 'this' before 'name' inside greet() method -> Option A

4. Quick Check:

   Use this.name to access property [OK]

Hint: Use 'this.' to access class properties inside methods [OK]

Common Mistakes:

• Forgetting 'this' keyword
• Thinking constructor syntax is wrong
• Believing method must return instead of console.log

5. You want to create many similar objects representing books with title and author. Why is using a class better than creating separate objects manually?

hard

A. Classes make the code run faster than objects

B. Classes let you create many objects easily with shared structure and behavior

C. Classes automatically save objects to a database

D. Classes prevent any changes to the objects after creation

Solution

1. Step 1: Understand the benefit of classes for similar objects

   Classes provide a blueprint to create many objects with the same properties and methods easily.

2. Step 2: Compare options with class benefits

   Only Classes let you create many objects easily with shared structure and behavior correctly states that classes simplify creating many similar objects with shared structure and behavior.

3. Final Answer:

   Classes let you create many objects easily with shared structure and behavior -> Option B

4. Quick Check:

   Classes simplify creating many similar objects = A [OK]

Hint: Classes are blueprints for many similar objects [OK]

Common Mistakes:

• Thinking classes speed up code execution
• Believing classes save data automatically
• Assuming classes make objects immutable