Prototype chain in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of accessing a property through the prototype chain?

Consider the following code. What will be logged to the console?

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 "Hello, Alice"

B undefined

C "Hello, undefined"

D TypeError: alice.greet is not a function

💡 Hint

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

The greet method is defined on Person.prototype, so alice inherits it. Calling alice.greet() returns 'Hello, Alice'.

❓ Predict Output

intermediate

What happens when a property is shadowed on the instance?

Look at this code. What will be the output of console.log(car.color)?

Javascript

function Vehicle() {}
Vehicle.prototype.color = 'red';

const car = new Vehicle();
car.color = 'blue';
console.log(car.color);

A "red"

B TypeError

C undefined

D "blue"

💡 Hint

Instance properties override prototype properties with the same name.

The instance car has its own color property set to 'blue', which shadows the prototype's 'red'. So car.color is 'blue'.

❓ Predict Output

advanced

What is the output when modifying the prototype after instance creation?

What will this code print to the console?

Javascript

function Animal() {}
Animal.prototype.sound = 'generic';

const dog = new Animal();
console.log(dog.sound);

Animal.prototype.sound = 'bark';
console.log(dog.sound);

A "generic" then "generic"

B "generic" then "bark"

C "bark" then "bark"

D TypeError

💡 Hint

Prototype properties are looked up dynamically at access time.

dog.sound first reads 'generic'. After changing Animal.prototype.sound to 'bark', dog.sound reflects the new value.

❓ Predict Output

advanced

What error occurs when accessing a missing property in the prototype chain?

What will happen when this code runs?

Javascript

const obj = {};
console.log(obj.someProperty.toString());

A TypeError: Cannot read properties of undefined (reading 'toString')

B undefined

C null

D ReferenceError

💡 Hint

Accessing a property on undefined causes an error.

obj.someProperty is undefined. Calling toString() on undefined causes a TypeError.

🧠 Conceptual

expert

Which prototype chain lookup order is correct?

Given an object instance, in what order does JavaScript look up properties in the prototype chain?

A Instance properties → Object.prototype → Constructor.prototype → null

B Constructor.prototype → Instance properties → Object.prototype → null

C Instance properties → Constructor.prototype → Object.prototype → null

D Object.prototype → Constructor.prototype → Instance properties → null

💡 Hint

Think about where properties are first checked: own properties or prototype.

JavaScript first checks own properties on the instance, then the constructor's prototype, then Object.prototype, then null.

Practice

1. What does the prototype chain in JavaScript help with?

easy

A. Sharing properties and methods between objects

B. Creating new variables automatically

C. Running code faster

D. Stopping code execution

Solution

1. Step 1: Understand prototype chain purpose

   The prototype chain allows objects to share properties and methods by linking them.

2. Step 2: Identify correct description

   Sharing properties and methods matches the prototype chain's role, unlike other options.

3. Final Answer:

   Sharing properties and methods between objects -> Option A

4. Quick Check:

   Prototype chain = Sharing properties and methods [OK]

Hint: Prototype chain shares properties/methods between objects [OK]

Common Mistakes:

• Thinking it creates variables
• Assuming it speeds up code
• Believing it stops code

2. Which of the following is the correct way to create an object with a specific prototype in JavaScript?

easy

A. let obj = prototypeObj.create();

B. let obj = Object.create(prototypeObj);

C. let obj = new Object(prototypeObj);

D. let obj = Object.prototype(prototypeObj);

Solution

1. Step 1: Recall Object.create syntax

   Object.create(prototypeObj) creates a new object with prototypeObj as its prototype.

2. Step 2: Check other options

   Other options use incorrect syntax or methods that don't exist.

3. Final Answer:

   let obj = Object.create(prototypeObj); -> Option B

4. Quick Check:

   Object.create() creates object with prototype [OK]

Hint: Use Object.create(prototype) to set prototype correctly [OK]

Common Mistakes:

• Using new Object() with prototype argument
• Calling create() on prototype object
• Trying to call Object.prototype as a function

3. What will be the output of this code?

const parent = { greet() { return 'Hello'; } };
const child = Object.create(parent);
console.log(child.greet());

medium

A. undefined

B. null

C. TypeError

D. 'Hello'

Solution

1. Step 1: Understand prototype lookup

   child has no greet method, so JavaScript looks up to parent where greet exists.

2. Step 2: Call greet method from parent

   Calling child.greet() runs parent.greet(), returning 'Hello'.

3. Final Answer:

   'Hello' -> Option D

4. Quick Check:

   Prototype lookup finds greet() = 'Hello' [OK]

Hint: If property missing, JS checks prototype chain [OK]

Common Mistakes:

• Expecting undefined because child lacks greet
• Thinking it causes error
• Assuming null is returned

4. Find the error in this code snippet:

const animal = { eats: true };
const rabbit = Object.create(animal);
rabbit.eats = false;
console.log(rabbit.eats);
console.log(animal.eats);

medium

A. No error; output is as expected

B. Object.create should not be used here

C. rabbit.eats does not override animal.eats

D. animal.eats is changed to false

Solution

1. Step 1: Understand property shadowing

   Setting rabbit.eats creates a new property on rabbit, shadowing animal.eats.

2. Step 2: Check outputs

   rabbit.eats logs false; animal.eats remains true. This is expected behavior.

3. Final Answer:

   No error; output is as expected -> Option A

4. Quick Check:

   Property shadowing works correctly [OK]

Hint: Setting property on child shadows parent's property [OK]

Common Mistakes:

• Thinking parent's property changes
• Believing Object.create is wrong here
• Expecting error from property override

5. You want to create a chain where child inherits from parent, and parent inherits from grandparent. Which code correctly sets this prototype chain?

hard

A. const grandparent = {};
const parent = Object.create(child);
const child = Object.create(parent);

B. const grandparent = {};
const parent = {};
const child = Object.create(parent);
child.__proto__ = grandparent;

C. const grandparent = {};
const parent = Object.create(grandparent);
const child = Object.create(parent);

D. const grandparent = {};
const parent = Object.create(grandparent);
const child = Object.create(grandparent);

Solution

1. Step 1: Set parent prototype to grandparent

   Using Object.create(grandparent) for parent sets grandparent as prototype.

2. Step 2: Set child prototype to parent

   Using Object.create(parent) for child sets parent as prototype, completing chain.

3. Step 3: Check other options

   B incorrectly overrides child's prototype to grandparent using __proto__, skipping parent; C creates a circular reference (child not defined when creating parent); D sets child directly to grandparent, skipping parent.

4. Final Answer:

   const grandparent = {}; const parent = Object.create(grandparent); const child = Object.create(parent); -> Option C

5. Quick Check:

   Prototype chain: child -> parent -> grandparent [OK]

Hint: Use Object.create in order from top to bottom [OK]

Common Mistakes:

• Setting child prototype to grandparent directly
• Reversing prototype chain order
• Using __proto__ instead of Object.create