Constructor functions in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a constructor function in JavaScript?

A constructor function is a special function used to create and initialize objects. It sets up properties and values for new objects using the new keyword.

beginner

How do you call a constructor function to create a new object?

You call a constructor function with the <code>new</code> keyword before the function name, like <code>const obj = new MyConstructor();</code>. This creates a new object and runs the constructor code.

beginner

What does the this keyword refer to inside a constructor function?

Inside a constructor function, this refers to the new object being created. You use this to add properties or methods to that new object.

intermediate

What happens if you forget to use new when calling a constructor function?

If you forget new, the constructor function runs like a normal function. this will not point to a new object, which can cause errors or unexpected results.

intermediate

How can you add methods to all objects created by a constructor function?

You add methods to the constructor's prototype. For example, MyConstructor.prototype.sayHi = function() { console.log('Hi'); };. All objects created will share this method.

What keyword do you use to create a new object with a constructor function?

A new

B create

C object

D construct

The new keyword creates a new object and runs the constructor function.

Inside a constructor function, what does this refer to?

A The constructor function itself

B The global object

C The new object being created

D Undefined

this points to the new object created by the constructor.

What happens if you call a constructor function without new?

A It returns null

B It throws a syntax error

C It creates a new object anyway

D It runs as a normal function, <code>this</code> is not the new object

Without new, the function runs normally and this does not refer to a new object.

How do you add a method shared by all objects created by a constructor?

A Add it to the constructor's prototype

B Use a global variable

C Add it to each object manually

D Add it inside the constructor function

Adding methods to the constructor's prototype shares them across all created objects.

Which of these is a correct constructor function name by convention?

A person

B Person

C personConstructor

D createPerson

Constructor functions usually start with a capital letter to distinguish them.

Explain how a constructor function works in JavaScript and how you create an object with it.

Think about how you make a new object and set its properties.

Describe how to add a method that all objects created by a constructor function can use.

Where do you put methods so all objects share them?

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