Constructors in classes in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a constructor in a JavaScript class?

A constructor is a special method in a class that runs automatically when a new object is created. It sets up the object with initial values.

beginner

How do you define a constructor inside a JavaScript class?

You define it using the keyword <code>constructor()</code> inside the class body. It looks like a regular method but named <code>constructor</code>.

beginner

What happens if you don't write a constructor in a JavaScript class?

If no constructor is written, JavaScript uses a default constructor that does nothing but allows object creation.

beginner

Can a JavaScript class have more than one constructor?

No, a class can only have one constructor method. Trying to add more will cause an error.

beginner

How do you use parameters in a constructor?

You add parameters inside the constructor() parentheses and use them to set properties on the new object.

What keyword is used to define a constructor in a JavaScript class?

A create

B init

C build

D constructor

The constructor method is defined using the keyword constructor inside the class.

What happens if you create an object from a class without a constructor?

A JavaScript throws an error

B A default constructor runs

C The object is not created

D You must add a constructor manually

If no constructor is defined, JavaScript uses a default constructor that allows object creation.

How many constructors can a JavaScript class have?

A As many as you want

B Two

C One

D None

A class can only have one constructor method.

Where do you put parameters to initialize object properties in a constructor?

A Inside the constructor parentheses

B After the class keyword

C Inside the class but outside the constructor

D In a separate method

Parameters go inside the constructor parentheses to set initial values.

Which of these is a correct constructor syntax in JavaScript?

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

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

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

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

The constructor is defined as a method inside the class without the function keyword.

Explain what a constructor does in a JavaScript class and why it is useful.

Think about how you prepare a new object with starting values.

Describe how you would add parameters to a constructor and use them to set object properties.

Consider how you pass information when creating a new object.

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