Why prototypes are needed in Javascript - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a prototype method that returns a greeting.

Javascript

function Person(name) {
  this.name = name;
}
Person.prototype.greet = function() {
  return [1] + this.name;
};

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

A'Hello, '

Bthis.name + 'Hello'

C'Hi ' + this.age

D'Goodbye, '

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong property like this.age instead of this.name

Putting the name before the greeting

The prototype method should return a greeting string followed by the person's name. Using 'Hello, ' + this.name achieves this.

2fill in blank

medium

Complete the code to add a shared property to all objects created by the constructor.

Javascript

function Car(model) {
  this.model = model;
}
Car.prototype.wheels = [1];

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

A0

B2

C4

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Setting wheels to 2 or 0 which is incorrect for cars

Leaving wheels undefined

Most cars have 4 wheels, so setting wheels to 4 on the prototype shares this property with all car objects.

3fill in blank

hard

Fix the error in the prototype method to correctly calculate age.

Javascript

function Person(birthYear) {
  this.birthYear = birthYear;
}
Person.prototype.getAge = function(currentYear) {
  return currentYear [1] this.birthYear;
};

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

A+

B*

C/

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition or multiplication which gives wrong results

Using division which is incorrect here

To calculate age, subtract the birth year from the current year.

4fill in blank

hard

Fill both blanks to create a prototype method that checks if a number is even.

Javascript

function NumberCheck(num) {
  this.num = num;
}
NumberCheck.prototype.isEven = function() {
  return this.num [1] 2 [2] 0;
};

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

A%

B===

C==

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using != instead of ===

Using == which is less strict than ===

The modulo operator (%) finds the remainder. If this.num % 2 === 0, the number is even.

5fill in blank

hard

Fill all three blanks to create a prototype method that returns a formatted description.

Javascript

function Book(title, author) {
  this.title = title;
  this.author = author;
}
Book.prototype.getDescription = function() {
  return [1] + ': ' + [2] + ' by ' + [3];
};

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

Athis.title

Bthis.author

Ctitle

Dauthor

Attempts:

3 left

💡 Hint

Common Mistakes

Using variable names without this

Mixing up title and author

The method should return the book's title followed by the author. Using this.title and this.author accesses the object's properties.

Practice

1. Why are prototypes needed in JavaScript?

easy

A. To prevent any changes to objects

B. To create new variables inside functions

C. To write code that runs faster on the server

D. To allow objects to share methods and properties efficiently

Solution

1. Step 1: Understand object sharing

   Prototypes let many objects share the same methods and properties without copying them.

2. Step 2: Recognize memory efficiency

   This sharing saves memory and makes code easier to maintain.

3. Final Answer:

   To allow objects to share methods and properties efficiently -> Option D

4. Quick Check:

   Prototypes = shared methods and properties [OK]

Hint: Prototypes share methods to save memory [OK]

Common Mistakes:

• Thinking prototypes create new variables
• Believing prototypes speed up server code
• Assuming prototypes prevent object changes

2. Which syntax correctly sets a method on a prototype in JavaScript?

easy

A. MyObject.myMethod = function() { return 1; };

B. MyObject.prototype.myMethod = function() { return 1; };

C. prototype.MyObject.myMethod = function() { return 1; };

D. MyObject->prototype->myMethod = function() { return 1; };

Solution

1. Step 1: Identify correct prototype syntax

   In JavaScript, the prototype property is accessed as MyObject.prototype.

2. Step 2: Confirm method assignment

   Assigning a function to MyObject.prototype.myMethod adds it to the prototype.

3. Final Answer:

   MyObject.prototype.myMethod = function() { return 1; }; -> Option B

4. Quick Check:

   Correct prototype syntax = MyObject.prototype.myMethod = function() { return 1; }; [OK]

Hint: Use .prototype to add methods to objects [OK]

Common Mistakes:

• Using MyObject.myMethod instead of prototype
• Writing prototype.MyObject instead of MyObject.prototype
• Using arrows (->) instead of dots (.)

3. What will be the output of this code?

function Person(name) { this.name = name; }
Person.prototype.greet = function() { return 'Hi, ' + this.name; };
const p = new Person('Anna');
console.log(p.greet());

medium

A. 'Hi, Anna'

B. ReferenceError: name is not defined

C. TypeError: p.greet is not a function

D. 'Hi, undefined'

Solution

1. Step 1: Understand constructor and prototype method

   The Person function sets this.name to 'Anna'. The greet method is on Person.prototype.

2. Step 2: Call greet on instance

   Calling p.greet() finds greet on prototype and returns 'Hi, Anna'.

3. Final Answer:

   'Hi, Anna' -> Option A

4. Quick Check:

   Prototype method returns greeting with name [OK]

Hint: Prototype methods are accessible on instances [OK]

Common Mistakes:

• Expecting greet to be undefined
• Confusing instance properties with prototype
• Thinking greet is not a function

4. Find the error in this code that tries to add a method to a prototype:

function Car() {}
Car.prototype = {
  start: function() { return 'Car started'; }
};
const c = new Car();
console.log(c.start());

medium

A. The prototype is overwritten, losing the constructor reference

B. The start method is not a function

C. Car is not defined before use

D. Cannot assign an object to prototype

Solution

1. Step 1: Check prototype assignment

   Assigning an object to Car.prototype replaces the original prototype and removes the constructor link.

2. Step 2: Understand consequence

   This can cause issues if code relies on constructor property or inheritance chain.

3. Final Answer:

   The prototype is overwritten, losing the constructor reference -> Option A

4. Quick Check:

   Overwriting prototype removes constructor [OK]

Hint: Avoid overwriting prototype object directly [OK]

Common Mistakes:

• Thinking start is not a function
• Believing Car is undefined
• Assuming prototype cannot be an object

5. You want to add a method describe to all objects created by function Animal(type) without copying it to each instance. How should you do it?

hard

A. Add describe method to each instance manually after creation

B. Add describe method inside Animal function using this.describe

C. Add describe method to Animal.prototype

D. Add describe method to Object.prototype

Solution

1. Step 1: Understand method sharing

   Adding describe to Animal.prototype shares it with all instances without copying.

2. Step 2: Avoid inefficient or unsafe options

   Adding inside constructor copies method per instance; adding to Object.prototype affects all objects globally.

3. Final Answer:

   Add describe method to Animal.prototype -> Option C

4. Quick Check:

   Use prototype to share methods efficiently [OK]

Hint: Put shared methods on constructor's prototype [OK]

Common Mistakes:

• Adding methods inside constructor wastes memory
• Modifying Object.prototype can cause bugs
• Manually adding methods to each instance is inefficient