Chapter 11 — Arrays

Q1. Suppose you're building an E-commerce website.

Your manager says:

"Store multiple product names instead of creating 100 variables."

Answer

Without Array ❌

let p1 = "Laptop";
let p2 = "Mouse";
let p3 = "Keyboard";

Using Array ✅

let products = ["Laptop", "Mouse", "Keyboard"];

console.log(products);

Output

["Laptop", "Mouse", "Keyboard"]

Accessing elements

console.log(products[0]);
console.log(products[2]);

Output

Laptop
Keyboard

Arrays use 0-based indexing.

Q2. Suppose your manager says:

"A new product arrived. Add it to the list."

Answer

let products = ["Laptop", "Mouse"];

products.push("Keyboard");

console.log(products);

Output

["Laptop", "Mouse", "Keyboard"]

Remove last product

products.pop();

console.log(products);

Output

["Laptop", "Mouse"]

Concepts

push() → Add at end

pop() → Remove from end

Q3. Suppose the first product becomes unavailable.

Your manager says:

"Remove the first product and add a new one at the beginning."

Answer

let products = ["Laptop", "Mouse", "Keyboard"];

products.shift();

console.log(products);

products.unshift("Tablet");

console.log(products);

Output

["Mouse", "Keyboard"]

["Tablet", "Mouse", "Keyboard"]

Concepts

shift() → Remove first

unshift() → Add first

Q4. Suppose interviewer asks:

"Difference between slice() and splice()?"

Answer

let arr = [10,20,30,40,50];

console.log(arr.slice(1,4));

Output

[20,30,40]

Original Array

[10,20,30,40,50]

Now

arr.splice(1,2);

console.log(arr);

Output

[10,40,50]

Difference

slice()	splice()
Doesn't modify original array	Modifies original array
Returns copied portion	Removes/Adds elements

Concepts Covered ✅

Arrays
Index
push()
pop()
shift()
unshift()
slice()
splice()
Accessing elements

Chapter 12 — Advanced Array Methods ⭐⭐⭐

Assume

let nums = [10,20,30,40,50];

Q1. Suppose your manager says:

"Increase every salary by ₹1000."

Answer

Use map()

let salary = [10000,20000,30000];

let updated = salary.map(s => s + 1000);

console.log(updated);

Output

[11000,21000,31000]

Why map?

Creates a new array.

Original remains unchanged.

Q2. Suppose manager says:

"Show only employees earning more than ₹20,000."

Answer

let salary = [10000,25000,15000,40000];

let result = salary.filter(s => s > 20000);

console.log(result);

Output

[25000,40000]

Why filter?

Keeps only matching values.

Q3. Suppose manager says:

"Calculate total sales."

Answer

let sales = [100,200,300,400];

let total = sales.reduce((sum,current)=>{

return sum + current;

},0);

console.log(total);

Output

Why reduce?

Reduces many values into one.

Q4. Suppose interviewer asks:

"Difference among map(), filter(), find(), some(), every(), forEach()?"

Answer

let nums=[10,20,30,40];

Find first value

console.log(nums.find(n=>n>20));

Output

Check if any value >30

console.log(nums.some(n=>n>30));

Output

true

Check all values >5

console.log(nums.every(n=>n>5));

Output

true

Loop

nums.forEach(n=>console.log(n));

Output

Summary

Method	Returns
map()	New transformed array
filter()	Matching elements
reduce()	Single value
find()	First matching element
some()	true/false
every()	true/false
forEach()	Nothing (just loops)

Concepts Covered ✅

map()
filter()
reduce()
find()
some()
every()
forEach()
Functional Programming Basics

Chapter 13 — Scope & Closures ⭐⭐⭐

Q1. Suppose your manager says:

"The variable salary should only be accessible inside a function."

Answer

function employee(){

let salary = 50000;

console.log(salary);

}

employee();

Output

Now

console.log(salary);

Output

ReferenceError

This is Local Scope.

Q2. Suppose interviewer asks:

"Difference between Global Scope and Block Scope?"

Answer

let company = "Google";

if(true){

let city = "Delhi";

}

console.log(company);

console.log(city);

Output

Google

ReferenceError

Scope Types

Scope	Accessible Where?
Global	Everywhere
Function	Inside function
Block	Inside `{}`

Q3. Suppose interviewer asks:

"What is Closure?"

Answer

function outer(){

let count = 0;

return function(){

count++;

console.log(count);

}

}

let counter = outer();

counter();

counter();

counter();

Output

Why?

Inner function remembers variables from outer function even after outer has finished.

That's Closure.

Q4. Suppose interviewer asks:

"Real-world use of Closure?"

Answer

Private Counter

function bank(){

let balance = 1000;

return {

check(){

console.log(balance);

},

deposit(amount){

balance += amount;

}

}

}

let account = bank();

account.deposit(500);

account.check();

Output

No one can directly do

balance = 0;

Closure provides Data Privacy.

Concepts Covered ✅

Global Scope
Local Scope
Block Scope
Lexical Scope
Closures
Data Hiding
Private Variables

Chapter 14 — Execution Context & Hoisting ⭐⭐⭐

Q1. Suppose interviewer asks:

"Why does this work?"

sayHello();

function sayHello(){

console.log("Hello");

}

Output

Hello

Answer

Function Declarations are Hoisted.

JavaScript moves them to memory before execution.

Q2. Suppose interviewer asks:

"Why does this fail?"

console.log(age);

let age = 22;

Output

ReferenceError

Why?

Because of

Temporal Dead Zone (TDZ)

let and const exist but can't be used before declaration.

Q3. Suppose interviewer asks:

"Difference between var and let hoisting?"

Answer

console.log(a);

var a = 10;

Output

undefined

Now

console.log(b);

let b = 10;

Output

ReferenceError

Q4. Suppose interviewer asks:

"Explain Execution Context."

Answer

When JavaScript runs

let x = 10;

let y = 20;

console.log(x+y);

JavaScript performs

1.

Memory Creation Phase

↓

2.

Execution Phase

During memory phase

Variables allocated

Functions stored

During execution

Assignments happen

Statements execute

Concepts Covered ✅

Execution Context
Memory Phase
Execution Phase
Hoisting
TDZ
var Hoisting
let Hoisting
Function Hoisting

Chapter 15 — Callbacks

Q1. Suppose your manager says:

"Download a file. After download completes, process it."

Answer

function download(callback){

console.log("Downloading...");

callback();

}

download(function(){

console.log("Processing File");

});

Output

Downloading...

Processing File

A function passed into another function is called a Callback.

Q2. Suppose you're building a Login System.

Manager says:

"Login first. Then load dashboard."

Answer

function login(callback){

console.log("Login Success");

callback();

}

function dashboard(){

console.log("Dashboard Loaded");

}

login(dashboard);

Output

Login Success

Dashboard Loaded

Q3. Suppose your project becomes like this.

Answer

login(function(){

loadProfile(function(){

loadPosts(function(){

loadComments(function(){

console.log("Done");

});

});

});

});

This is called

Callback Hell

Problems

Difficult to read
Difficult to debug
Difficult to maintain

Promises solve this problem.

Q4. Suppose interviewer asks:

"When do we use callbacks?"

Answer

Whenever an operation finishes later, and another operation should start after it.

Examples:

Reading files
API calls
Database queries
Button click events
Timers (setTimeout)
User authentication

Example with setTimeout:

console.log("Start");

setTimeout(() => {
    console.log("Task Finished");
}, 2000);

console.log("End");

Output

Start
End
Task Finished

Why?

setTimeout is asynchronous. JavaScript doesn't wait—it continues executing the next statement and runs the callback after the timer completes.

Concepts Covered ✅

Callback Functions
Passing Functions as Arguments
Callback Execution
Asynchronous Programming Basics
Callback Hell
Real-world Callback Use Cases
Introduction to setTimeout

🎯 After Chapters 11–15, you've reached intermediate JavaScript.

You now understand:

Arrays and their most common operations.
Powerful array methods (map, filter, reduce, etc.).
Scope and closures—two of the most important interview topics.
Execution context, hoisting, and the Temporal Dead Zone.
Callback functions and why asynchronous programming needs them.

The next chapters (16–18) will cover Promises, Async/Await, and the Event Loop—the core concepts behind modern asynchronous JavaScript.