Constructor functions in Javascript - Mini Project: Build & Apply

Building Objects with Constructor Functions

📖 Scenario: You are creating a simple system to keep track of books in a library. Each book has a title and an author.

🎯 Goal: You will build a constructor function to create book objects, then create a few books and display their details.

📋 What You'll Learn

Create a constructor function named Book that takes title and author as parameters

Create a variable named book1 using the Book constructor with title 'The Hobbit' and author 'J.R.R. Tolkien'

Create a variable named book2 using the Book constructor with title '1984' and author 'George Orwell'

Print the title and author of book1 and book2 using console.log

💡 Why This Matters

🌍 Real World

Constructor functions help create many similar objects easily, like books, users, or products in real applications.

💼 Career

Understanding constructor functions is important for JavaScript developers to organize and reuse code efficiently when working with objects.

1

Create the Book constructor function

Write a constructor function named Book that takes two parameters: title and author. Inside the function, assign this.title to title and this.author to author.

Javascript

// Your code here

Hint

Use function Book(title, author) { ... } and inside assign this.title = title and this.author = author.

2

Create two book objects using the Book constructor

Create a variable named book1 and assign it a new Book object with title 'The Hobbit' and author 'J.R.R. Tolkien'. Then create a variable named book2 and assign it a new Book object with title '1984' and author 'George Orwell'.

Javascript

function Book(title, author) {
    this.title = title;
    this.author = author;
}

// Your code here

Hint

Use const book1 = new Book('The Hobbit', 'J.R.R. Tolkien'); and similarly for book2.

3

Print the details of the books

Use console.log to print the title and author of book1 and book2. For example, print book1.title and book1.author on one line, and book2.title and book2.author on another line.

Javascript

function Book(title, author) {
    this.title = title;
    this.author = author;
}

const book1 = new Book('The Hobbit', 'J.R.R. Tolkien');
const book2 = new Book('1984', 'George Orwell');

// Your code here

Hint

Use console.log(book1.title + ' by ' + book1.author); and similarly for book2.

4

Run the program to see the output

Run the program and observe the output. It should print the title and author of both books on separate lines.

Javascript

function Book(title, author) {
    this.title = title;
    this.author = author;
}

const book1 = new Book('The Hobbit', 'J.R.R. Tolkien');
const book2 = new Book('1984', 'George Orwell');

console.log(book1.title + ' by ' + book1.author);
console.log(book2.title + ' by ' + book2.author);

// Run the program to see the output

Hint

The output should show the book titles and authors exactly as:
The Hobbit by J.R.R. Tolkien
1984 by George Orwell

Practice

1. What does the new keyword do when used with a constructor function in JavaScript?

easy

A. Creates a new object and sets this to that object inside the constructor

B. Calls the constructor function without creating a new object

C. Deletes the constructor function after running

D. Returns a string representation of the constructor

Solution

1. Step 1: Understand the role of new

   The new keyword creates a fresh object and sets this inside the constructor to that object.

2. Step 2: Effect on constructor function

   It allows the constructor to initialize properties on the new object and then returns it automatically.

3. Final Answer:

   Creates a new object and sets this to that object inside the constructor -> Option A

4. Quick Check:

   new creates object = A [OK]

Hint: Remember: new makes a new object and sets this [OK]

Common Mistakes:

• Thinking new just calls the function without creating an object
• Believing new returns a string
• Confusing new with deleting functions

2. Which of the following is the correct syntax to define a constructor function named Car?

easy

A. function Car() { this.make = 'Toyota'; }

B. var Car = () => { this.make = 'Toyota'; }

C. class Car { make = 'Toyota'; }

D. function Car[] { this.make = 'Toyota'; }

Solution

1. Step 1: Identify valid constructor function syntax

   A constructor function is a normal function with a capitalized name, like function Car() {}.

2. Step 2: Check each option

   function Car() { this.make = 'Toyota'; } uses correct function syntax. var Car = () => { this.make = 'Toyota'; } uses arrow function which does not bind this properly. class Car { make = 'Toyota'; } is a class, not a constructor function. function Car[] { this.make = 'Toyota'; } has invalid syntax with square brackets.

3. Final Answer:

   function Car() { this.make = 'Toyota'; } -> Option A

4. Quick Check:

   Constructor function = normal function with capital name [OK]

Hint: Use normal function with capital name for constructor [OK]

Common Mistakes:

• Using arrow functions as constructors
• Trying to use class syntax instead of function
• Writing invalid function syntax with brackets

3. What will be the output of this code?

function Person(name) {
  this.name = name;
}
const p = new Person('Alice');
console.log(p.name);

medium

A. undefined

B. Error

C. Alice

D. Person

Solution

1. Step 1: Understand object creation with constructor

   The new Person('Alice') creates a new object and sets this.name to 'Alice'.

2. Step 2: Access the property

   Logging p.name prints the value assigned, which is 'Alice'.

3. Final Answer:

   Alice -> Option C

4. Quick Check:

   Property set by constructor = 'Alice' [OK]

Hint: Remember: this.name sets property on new object [OK]

Common Mistakes:

• Expecting undefined because of missing return
• Confusing constructor name with property value
• Thinking console.log prints function name

4. Find the error in this constructor function:

function Animal(type) {
  type = type;
  this.type = type;
}
const a = new Animal('Dog');
console.log(a.type);

medium

A. Missing new keyword when creating object

B. Cannot assign this.type inside constructor

C. Constructor function name should be lowercase

D. The assignment type = type; does nothing and is redundant

Solution

1. Step 1: Analyze the assignment type = type;

   This line assigns the parameter type to itself, which does nothing and is unnecessary.

2. Step 2: Check other parts

   The line this.type = type; correctly assigns the parameter to the object property. The new keyword is used correctly. Function name capitalization is fine.

3. Final Answer:

   The assignment type = type; does nothing and is redundant -> Option D

4. Quick Check:

   Redundant self-assignment = B [OK]

Hint: Check if variables are assigned to themselves unnecessarily [OK]

Common Mistakes:

• Thinking type = type; sets object property
• Forgetting to use new keyword
• Believing constructor names must be lowercase

5. You want to create a constructor function Book that takes title and author as parameters and adds a method describe that returns a string like "Title by Author". Which code correctly does this?

hard

A. function Book(title, author) {
this.title = title;
this.author = author;
}
Book.describe = function() {
return `${this.title} by ${this.author}`;
};

B. function Book(title, author) {
this.title = title;
this.author = author;
this.describe = function() {
return `${this.title} by ${this.author}`;
};
}

C. function Book(title, author) {
this.title = title;
this.author = author;
describe() {
return `${this.title} by ${this.author}`;
}
}

D. function Book(title, author) {
this.title = title;
this.author = author;
this.describe = `${this.title} by ${this.author}`;
}

Solution

1. Step 1: Check method definition inside constructor

   function Book(title, author) { this.title = title; this.author = author; this.describe = function() { return `${this.title} by ${this.author}`; }; } correctly defines describe as a function property on this, so each object has its own method.

2. Step 2: Analyze other options

   function Book(title, author) { this.title = title; this.author = author; } Book.describe = function() { return `${this.title} by ${this.author}`; }; adds describe to the constructor function itself, not instances. function Book(title, author) { this.title = title; this.author = author; describe() { return `${this.title} by ${this.author}`; } } has invalid syntax for method inside function. function Book(title, author) { this.title = title; this.author = author; this.describe = `${this.title} by ${this.author}`; } assigns a string, not a function, so describe is not callable.

3. Final Answer:

   function Book(title, author) { this.title = title; this.author = author; this.describe = function() { return `${this.title} by ${this.author}`; }; } -> Option B

4. Quick Check:

   Method as function property on this = A [OK]

Hint: Define methods as functions on this inside constructor [OK]

Common Mistakes:

• Adding methods to constructor function instead of instances
• Using invalid syntax for methods inside functions
• Assigning method as string instead of function