Constructors in classes in Javascript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a constructor in the class.

Javascript

class Person {
  constructor([1]) {
    this.name = name;
  }
}

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

Aname

Bage

Cthis

Dconstructor

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'this' as a parameter name.

Using 'constructor' as a parameter name.

Using a parameter unrelated to the property.

The constructor needs a parameter name to assign it to the object.

2fill in blank

medium

Complete the code to create a new instance of the class.

Javascript

const person1 = new Person([1]);

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

APerson

Bthis

Cconstructor

D'John'

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the class name instead of a string.

Passing 'constructor' or 'this' instead of a string.

Not passing any argument.

To create a new object, pass the name string to the constructor.

3fill in blank

hard

Fix the error in the constructor parameter to correctly assign the property.

Javascript

class Animal {
  constructor([1]) {
    this.type = type;
  }
}

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

Athis.type

Btype

CanimalType

Dtype()

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'this.type' as a parameter.

Using a function call 'type()' as a parameter.

Using a different parameter name without updating the assignment.

The constructor parameter should be type to assign it to this.type.

4fill in blank

hard

Fill both blanks to create a class with a constructor that sets two properties.

Javascript

class Car {
  constructor([1], [2]) {
    this.make = make;
    this.year = year;
  }
}

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

Amake

Bmodel

Cyear

Dcolor

Attempts:

3 left

💡 Hint

Common Mistakes

Using parameters that do not match the property names.

Using unrelated parameter names like 'model' or 'color'.

The constructor needs parameters make and year to assign to the object properties.

5fill in blank

hard

Fill all three blanks to create a class with a constructor and a method that returns a description.

Javascript

class Book {
  constructor([1], [2]) {
    this.title = title;
    this.author = author;
  }

  getDescription() {
    return `$[3] by ${this.author}`;
  }
}

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

Atitle

Bauthor

Cthis.title

Dtitle()

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'title()' instead of 'this.title' in the method.

Using wrong parameter names in the constructor.

Not using 'this' to access object properties.

The constructor parameters are title and author. The method uses this.title to return the description.

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