Prototype inheritance in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of prototype property access

What is the output of the following code?

Javascript

function Person(name) {
  this.name = name;
}
Person.prototype.greet = function() {
  return `Hello, ${this.name}`;
};
const alice = new Person('Alice');
console.log(alice.greet());

A TypeError: alice.greet is not a function

B undefined

C "Hello, Alice"

D "Hello, undefined"

💡 Hint

Remember that methods defined on the prototype are accessible to instances.

The greet method is defined on Person.prototype, so alice inherits it and can call it. The name property is set to 'Alice', so the output is "Hello, Alice".

❓ Predict Output

intermediate

Effect of changing prototype after instance creation

What will be the output of this code?

Javascript

function Animal() {}
Animal.prototype.sound = function() { return 'generic sound'; };
const dog = new Animal();
Animal.prototype = {
  sound: function() { return 'bark'; }
};
console.log(dog.sound());

A "bark"

B "generic sound"

C TypeError: dog.sound is not a function

D undefined

💡 Hint

Think about when the prototype is linked to the instance.

The dog instance keeps the original prototype linked at creation. Changing Animal.prototype later does not affect existing instances. So dog.sound() calls the original method returning "generic sound".

🔧 Debug

advanced

Why does this prototype chain fail?

The following code tries to create a prototype chain where Student inherits from Person. Why does calling student.introduce() cause an error?

Javascript

function Person(name) {
  this.name = name;
}
Person.prototype.introduce = function() {
  return `Hi, I'm ${this.name}`;
};

function Student(name, subject) {
  Person.call(this, name);
  this.subject = subject;
}
Student.prototype = Person;
Student.prototype.constructor = Student;

const student = new Student('Bob', 'Math');
console.log(student.introduce());

A undefined

B "Hi, I'm Bob"

C ReferenceError: introduce is not defined

D TypeError: student.introduce is not a function

💡 Hint

Check how the prototype is assigned for Student.

Assigning Student.prototype = Person assigns the function Person itself, not its prototype object. So student.__proto__ is a function, which does not have introduce method. The correct way is Student.prototype = Object.create(Person.prototype).

🧠 Conceptual

advanced

Prototype chain property lookup order

Given the following objects, which property value will be logged?

Javascript

const grandparent = { trait: 'wisdom' };
const parent = Object.create(grandparent);
parent.trait = 'strength';
const child = Object.create(parent);
console.log(child.trait);

A "strength"

B "wisdom"

C undefined

D TypeError

💡 Hint

Remember how property lookup works in prototype chains.

child does not have trait, so it looks up to parent which has trait = 'strength'. It returns that value without checking grandparent.

❓ Predict Output

expert

Output of modifying prototype after instance creation with method override

What is the output of this code?

Javascript

function Vehicle() {}
Vehicle.prototype.move = function() { return 'moving'; };
const car = new Vehicle();
Vehicle.prototype.move = function() { return 'driving'; };
console.log(car.move());

A "driving"

B undefined

C TypeError: car.move is not a function

D "moving"

💡 Hint

Think about how methods on the prototype are looked up at call time.

car.move looks up the move method on Vehicle.prototype at call time. Since the method was replaced after car was created, car.move calls the new method returning "driving".

Practice

1. What does prototype inheritance allow in JavaScript?

easy

A. Arrays to store only numbers

B. Functions to run faster

C. Variables to be private

D. Objects to share properties and methods

Solution

1. Step 1: Understand prototype inheritance concept

   Prototype inheritance allows one object to use properties and methods from another object.

2. Step 2: Match concept to options

   Only Objects to share properties and methods correctly describes sharing properties and methods between objects.

3. Final Answer:

   Objects to share properties and methods -> Option D

4. Quick Check:

   Prototype inheritance = share properties and methods [OK]

Hint: Prototype inheritance means sharing properties/methods [OK]

Common Mistakes:

• Thinking it speeds up functions
• Confusing with variable privacy
• Assuming it limits array types

2. Which syntax correctly creates an object that inherits from another object parentObj?

easy

A. const child = parentObj.create();

B. const child = new Object(parentObj);

C. const child = Object.create(parentObj);

D. const child = Object.inherit(parentObj);

Solution

1. Step 1: Recall correct syntax for prototype inheritance

   Use Object.create(parentObj) to create a new object inheriting from parentObj.

2. Step 2: Check options for correct method

   Only const child = Object.create(parentObj); uses Object.create correctly; others are invalid or incorrect.

3. Final Answer:

   const child = Object.create(parentObj); -> Option C

4. Quick Check:

   Object.create() creates inherited object [OK]

Hint: Use Object.create(parent) to inherit prototype [OK]

Common Mistakes:

• Using new Object(parentObj) which doesn't inherit
• Using non-existent Object.inherit method
• Calling create on parentObj instead of Object

3. What is the output of this code?

function Parent() { this.name = 'Parent'; }
Parent.prototype.greet = function() { return 'Hello from ' + this.name; };

function Child() { Parent.call(this); this.name = 'Child'; }
Child.prototype = Object.create(Parent.prototype);
Child.prototype.constructor = Child;

const c = new Child();
console.log(c.greet());

medium

A. 'Hello from Parent'

B. 'Hello from Child'

C. undefined

D. TypeError: c.greet is not a function

Solution

1. Step 1: Understand property assignment in Child constructor

   Child calls Parent constructor to set this.name, then overwrites it with 'Child'.

2. Step 2: Check prototype chain and method call

   Child's prototype inherits from Parent.prototype, so greet() is available and uses this.name which is 'Child'.

3. Final Answer:

   'Hello from Child' -> Option B

4. Quick Check:

   Child instance uses own name 'Child' in greet() [OK]

Hint: Child overwrites name, greet uses Child's name [OK]

Common Mistakes:

• Assuming greet uses Parent's name
• Forgetting to call Parent.call(this)
• Not resetting Child.prototype.constructor

4. Find the error in this code that tries to set up prototype inheritance:

function Animal() { this.alive = true; }
Animal.prototype.breathe = function() { return 'breathing'; };

function Dog() { Animal.call(this); this.bark = function() { return 'woof'; } }
Dog.prototype = Animal.prototype;

const d = new Dog();
console.log(d.breathe());

medium

A. Dog.prototype should be Object.create(Animal.prototype)

B. Animal.call(this) should be Dog.call(this)

C. Dog.prototype.bark should be defined outside constructor

D. No error, code works fine

Solution

1. Step 1: Identify prototype assignment mistake

   Assigning Dog.prototype = Animal.prototype links both constructors to same prototype object, causing shared mutations.

2. Step 2: Correct way to inherit prototype

   Use Dog.prototype = Object.create(Animal.prototype) to create a new object inheriting from Animal.prototype without sharing it directly.

3. Final Answer:

   Dog.prototype should be Object.create(Animal.prototype) -> Option A

4. Quick Check:

   Use Object.create for prototype inheritance [OK]

Hint: Use Object.create to avoid shared prototype object [OK]

Common Mistakes:

• Assigning prototype directly instead of using Object.create
• Confusing call() usage
• Defining methods inside constructor unnecessarily

5. You want to create a Car object that inherits from Vehicle and adds a drive method. Which code correctly sets this up and calls drive on a Car instance?

hard

A. function Vehicle() { this.wheels = 4; }
Vehicle.prototype.move = function() { return 'moving'; };

function Car() { Vehicle.call(this); }
Car.prototype = Object.create(Vehicle.prototype);
Car.prototype.constructor = Car;
Car.prototype.drive = function() { return 'driving'; };

const myCar = new Car();
console.log(myCar.drive());

B. function Vehicle() { this.wheels = 4; }
Vehicle.prototype.move = function() { return 'moving'; };

function Car() { this.wheels = 4; }
Car.prototype = Vehicle.prototype;
Car.prototype.drive = function() { return 'driving'; };

const myCar = new Car();
console.log(myCar.drive());

C. function Vehicle() { this.wheels = 4; }
Vehicle.prototype.move = function() { return 'moving'; };

function Car() { Vehicle.call(this); }
Car.prototype.drive = function() { return 'driving'; };

const myCar = new Car();
console.log(myCar.drive());

D. function Vehicle() { this.wheels = 4; }
Vehicle.prototype.move = function() { return 'moving'; };

function Car() { Vehicle.call(this); }
Car.prototype = Object.create(Vehicle.prototype);

const myCar = new Car();
console.log(myCar.drive());

Solution

1. Step 1: Set up inheritance correctly

   Use Vehicle.call(this) inside Car constructor to copy properties, then set Car.prototype = Object.create(Vehicle.prototype) to inherit methods.

2. Step 2: Add drive method and create instance

   Define drive on Car.prototype, then create instance and call drive().

3. Final Answer:

   Code in option A correctly sets inheritance and calls drive() -> Option A

4. Quick Check:

   Call parent, use Object.create, add method, then call [OK]

Hint: Call parent, Object.create prototype, add method, then use instance [OK]

Common Mistakes:

• Assigning prototype directly instead of Object.create
• Not calling Vehicle.call(this) in Car
• Forgetting to set constructor property
• Defining methods before setting prototype