Constructors in classes in Javascript - Mini Project: Build & Apply

Constructors in Classes

📖 Scenario: You are creating a simple program to keep track of pets in a pet store. Each pet has a name and an age.

🎯 Goal: Build a class called Pet that uses a constructor to set the pet's name and age. Then create a pet object and display its details.

📋 What You'll Learn

Create a class named Pet

Add a constructor method with parameters name and age

Inside the constructor, assign name and age to the object properties

Create an instance of Pet with name Buddy and age 3

Print the pet's name and age using the instance

💡 Why This Matters

🌍 Real World

Classes with constructors are used to create objects that represent real things, like pets, users, or products, with their own data.

💼 Career

Understanding constructors is essential for building organized and reusable code in many programming jobs, especially in web development.

1

Create the Pet class

Write a class called Pet with an empty constructor method.

Javascript

class Pet {
  constructor() {
    // Your code here
  }
}

Hint

Use the class keyword followed by Pet. Inside, add constructor() with empty braces.

2

Add parameters and assign properties in the constructor

Modify the constructor to accept parameters name and age. Inside the constructor, assign this.name = name and this.age = age.

Javascript

class Pet {
  constructor(name, age) {
    // Your code here
  }
}

Hint

Inside the constructor, use this.name = name and this.age = age to store the values.

3

Create a Pet instance

Create a variable called myPet and assign it a new Pet object with name "Buddy" and age 3.

Javascript

class Pet {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }
}

// Your code here

Hint

Use new Pet("Buddy", 3) to create the object and assign it to myPet.

4

Print the pet's name and age

Write a console.log statement to print myPet.name and myPet.age separated by a space.

Javascript

class Pet {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }
}

const myPet = new Pet("Buddy", 3);

// Your code here

Hint

Use console.log(myPet.name + " " + myPet.age) to print the values.

Practice

1. What is the main purpose of a constructor in a JavaScript class?

easy

A. To create static properties for the class

B. To define methods that all objects share

C. To initialize new objects with default or given values

D. To delete objects when they are no longer needed

Solution

1. Step 1: Understand the role of a constructor

   A constructor is a special method in a class that runs automatically when a new object is created.

2. Step 2: Identify what constructors do

   Constructors set initial values for the object's properties using this.

3. Final Answer:

   To initialize new objects with default or given values -> Option C

4. Quick Check:

   Constructor = initialize object [OK]

Hint: Constructors always set up new objects [OK]

Common Mistakes:

• Confusing constructors with regular methods
• Thinking constructors delete objects
• Believing constructors create static properties

2. Which of the following is the correct syntax to define a constructor inside a JavaScript class?

easy

A. constructor() { this.name = 'John'; }

B. function constructor() { this.name = 'John'; }

C. def constructor() { this.name = 'John'; }

D. constructor = () => { this.name = 'John'; }

Solution

1. Step 1: Recall JavaScript class constructor syntax

   In JavaScript classes, the constructor is defined as a method named constructor without the function keyword.

2. Step 2: Check each option

   constructor() { this.name = 'John'; } uses the correct syntax: constructor() { ... }. Options B and C incorrectly use keywords from other languages or function syntax. constructor = () => { this.name = 'John'; } uses arrow function syntax, which is invalid for constructors.

3. Final Answer:

   constructor() { this.name = 'John'; } -> Option A

4. Quick Check:

   Constructor method = constructor() { } [OK]

Hint: Constructor is a method named exactly 'constructor' [OK]

Common Mistakes:

• Adding 'function' keyword before constructor
• Using arrow functions for constructors
• Using other language syntax like 'def'

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

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

medium

A. Hello, undefined!

B. Hello, Alice!

C. Hello, !

D. TypeError: p.greet is not a function

Solution

1. Step 1: Understand object creation with constructor

   The Person class constructor sets this.name to the passed argument. Here, 'Alice' is passed, so p.name becomes 'Alice'.

2. Step 2: Evaluate the greet method output

   The greet method returns a string using this.name, which is 'Alice'. So, p.greet() returns 'Hello, Alice!'.

3. Final Answer:

   Hello, Alice! -> Option B

4. Quick Check:

   Constructor sets name, greet uses it = Hello, Alice! [OK]

Hint: Constructor sets property, method uses it [OK]

Common Mistakes:

• Forgetting to pass argument to constructor
• Expecting undefined because of missing this
• Thinking greet is not a method

4. Identify the error in this class definition:

class Car {
  constructor(make, model) {
    make = make;
    model = model;
  }
}

medium

A. Properties should be assigned using this, like this.make = make;

B. The class name should be lowercase

C. The constructor should not have parameters

D. The constructor must return an object explicitly

Solution

1. Step 1: Review property assignment in constructor

   Inside a constructor, to assign values to the object's properties, you must use this.propertyName. Simply writing make = make; assigns the parameter to itself, which does nothing.

2. Step 2: Identify the correct fix

   Using this.make = make; and this.model = model; correctly assigns the values to the new object's properties.

3. Final Answer:

   Properties should be assigned using this, like this.make = make; -> Option A

4. Quick Check:

   Use this.property = parameter [OK]

Hint: Always use this.property to assign inside constructor [OK]

Common Mistakes:

• Assigning parameters to themselves
• Not using this to set properties
• Thinking constructor must return explicitly

5. You want to create a Book class where each book has a title and an optional author. If no author is given, it should default to 'Unknown'. Which constructor correctly implements this behavior?

hard

A. constructor(title, author) { this.title = title; if (author) this.author = author; }

B. constructor(title, author) { this.title = title; this.author = author || 'Unknown'; }

C. constructor(title) { this.title = title; this.author = 'Unknown'; }

D. constructor(title, author = 'Unknown') { this.title = title; this.author = author; }

Solution

1. Step 1: Understand optional parameters in constructors

   Use default parameter values for optional arguments. constructor(title, author = 'Unknown') { this.title = title; this.author = author; }: constructor(title, author = 'Unknown') sets author to 'Unknown' if not provided or undefined, but keeps passed falsy values like empty string.

2. Step 2: Check other options

   B uses ||, which defaults falsy values (not just missing). C has no author param. D sets only if truthy, else undefined.

3. Final Answer:

   constructor(title, author = 'Unknown') { this.title = title; this.author = author; } -> Option D

4. Quick Check:

   Optional param: param = default [OK]

Hint: Default params for optional constructor args [OK]

Common Mistakes:

• Using logical OR (||) which treats falsy as missing
• Omitting the optional parameter entirely
• Not providing a default, leaving undefined