Prototype chain in Javascript - Mini Project: Build & Apply

Understanding Prototype Chain in JavaScript

📖 Scenario: Imagine you have a simple object representing a car. You want to understand how JavaScript looks up properties through the prototype chain when you try to access a property that is not directly on the object.

🎯 Goal: You will create an object with a prototype, add properties to both, and then access properties to see how the prototype chain works.

📋 What You'll Learn

Create an object called car with a property make set to 'Toyota'

Create an object called vehicle with a property wheels set to 4

Set vehicle as the prototype of car

Access and print the make property from car

Access and print the wheels property from car to show prototype chain lookup

💡 Why This Matters

🌍 Real World

Understanding prototype chains helps you work with JavaScript objects and inheritance, which is common in web development and frameworks.

💼 Career

Many JavaScript jobs require knowledge of prototypes to debug code, optimize performance, and understand how libraries and frameworks work under the hood.

1

Create the car object

Create an object called car with a property make set to 'Toyota'.

Javascript

// Create the car object with make property
// Your code here

Hint

Use const car = { make: 'Toyota' }; to create the object.

2

Create the vehicle object

Create an object called vehicle with a property wheels set to 4.

Javascript

const car = { make: 'Toyota' };
// Create the vehicle object with wheels property
// Your code here

Hint

Use const vehicle = { wheels: 4 }; to create the object.

3

Set vehicle as the prototype of car

Use Object.setPrototypeOf to set vehicle as the prototype of car.

Javascript

const car = { make: 'Toyota' };
const vehicle = { wheels: 4 };
// Set vehicle as the prototype of car
// Your code here

Hint

Use Object.setPrototypeOf(car, vehicle); to link the prototype.

4

Access and print properties to see prototype chain

Print the make property from car and then print the wheels property from car to show prototype chain lookup.

Javascript

const car = { make: 'Toyota' };
const vehicle = { wheels: 4 };
Object.setPrototypeOf(car, vehicle);
// Print car.make
// Print car.wheels
// Your code here

Hint

Use console.log(car.make); and console.log(car.wheels); to print the properties.

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