Constructor functions in Javascript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a constructor function named Person.

Javascript

function [1](name, age) {
  this.name = name;
  this.age = age;
}

Drag options to blanks, or click blank then click option'

AcreatePerson

Bperson

CPerson

DnewPerson

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase function names for constructors.

Using names that do not describe the object.

The constructor function must be named Person to match the usage convention.

2fill in blank

medium

Complete the code to create a new Person object named john.

Javascript

const john = new [1]('John', 30);

Drag options to blanks, or click blank then click option'

AnewPerson

BPerson

CcreatePerson

Dperson

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase names when calling the constructor.

Forgetting the new keyword.

Use the Person constructor function with the new keyword to create an object.

3fill in blank

hard

Fix the error in the constructor function to correctly assign the age property.

Javascript

function Person(name, age) {
  this.name = name;
  this.[1] = age;
}

Drag options to blanks, or click blank then click option'

Aages

Bagee

CAge

Dage

Attempts:

3 left

💡 Hint

Common Mistakes

Misspelling the property name.

Using uppercase letters incorrectly.

The property name must exactly match age to store the age value correctly.

4fill in blank

hard

Fill both blanks to add a method greet to the Person prototype that returns a greeting string.

Javascript

Person.prototype.[1] = function() {
  return `Hello, my name is [2].`;
};

Drag options to blanks, or click blank then click option'

Agreet

BsayHello

Cthis.name

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect method names.

Not using this to access properties.

The method name should be greet, and inside the method, use this.name to access the object's name.

5fill in blank

hard

Fill all three blanks to create a Person object and call its greet method, storing the result in message.

Javascript

const [1] = new Person('Alice', 25);
const [2] = [3].greet();

Drag options to blanks, or click blank then click option'

Aalice

Bmessage

Dgreet

Attempts:

3 left

💡 Hint

Common Mistakes

Using inconsistent variable names.

Forgetting to call the method.

Create an object named alice, then call greet() on it and store the result in message.

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