Constructor functions in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple constructor function

What is the output of the following code?

Javascript

function Person(name, age) {
  this.name = name;
  this.age = age;
}

const p = new Person('Alice', 30);
console.log(p.name + ' is ' + p.age + ' years old.');

A TypeError

B "Alice is 30 years old."

C "Alice is undefined years old."

D "undefined is 30 years old."

💡 Hint

Remember that the new keyword creates a new object and sets this inside the function.

The constructor function assigns name and age to the new object. So p.name is 'Alice' and p.age is 30.

❓ Predict Output

intermediate

Effect of missing new keyword

What happens when you call a constructor function without the new keyword?

Javascript

function Car(make, model) {
  this.make = make;
  this.model = model;
}

const c = Car('Toyota', 'Corolla');
console.log(c);

A undefined

B An object with make and model properties

C TypeError

D ReferenceError

💡 Hint

Without new, the function does not return an object unless explicitly returned.

Calling Car without new runs the function but does not create or return an object. So c is undefined.

❓ Predict Output

advanced

Prototype method behavior

What is the output of this code?

Javascript

function Animal(type) {
  this.type = type;
}
Animal.prototype.speak = function() {
  return this.type + ' makes a sound';
};
const dog = new Animal('Dog');
console.log(dog.speak());

A TypeError

B "undefined makes a sound"

C "Dog makes a sound"

D "Animal makes a sound"

💡 Hint

Prototype methods are shared by all instances.

The speak method uses this.type which is 'Dog' for the dog instance.

🧠 Conceptual

advanced

Understanding constructor return values

What will be the value of obj after running this code?

Javascript

function Gadget(name) {
  this.name = name;
  return { name: 'Override' };
}
const obj = new Gadget('Original');

A { name: 'Original' }

B TypeError

C undefined

D { name: 'Override' }

💡 Hint

If a constructor returns an object explicitly, that object is used instead of this.

Returning an object from a constructor replaces the created object, so obj is the returned object.

🔧 Debug

expert

Fixing prototype method context error

What error will this code produce when calling greet, and why?

Javascript

function User(name) {
  this.name = name;
}
User.prototype.greet = () => {
  return 'Hello, ' + this.name;
};
const u = new User('Sam');
console.log(u.greet());

A "Hello, undefined"

B ReferenceError

C TypeError

D "Hello, Sam"

💡 Hint

Arrow functions do not have their own this; they use the surrounding context.

The arrow function uses the outer this, which is not the User instance, so this.name is undefined.

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