DEV Community: Raja B

Interview Question

Raja B — Thu, 25 Jun 2026 16:54:45 +0000

In JavaScript, var, let, and const are all used to declare variables, but they differ in scope, reassignment, redeclaration, and hoisting behavior:

1. Scope

var: Function-scoped — accessible anywhere inside the function it's declared (or globally if outside).
let & const: Block-scoped — only accessible inside the { } block (like inside an if, for, or function).

function test() {
  if (true) {
    var a = 1;
    let b = 2;
    const c = 3;
  }
  console.log(a); //  1 (works)
  console.log(b); //  Error: b is not defined
  console.log(c); //  Error: c is not defined
}
test();

2. Reassignment

let: You can change the value later
const: You cannot change the value after first assignment (must be initialized immediately)

let x = 5;
x = 10; //  OK

const y = 5;
y = 10; //  Error: Assignment to constant variable

Note: For objects with const, you can modify their properties (since the reference doesn't change), but you can't reassign the whole object.

3. Redeclaration

var: You can declare the same variable multiple times
let & const: Redeclaration causes an error

var a = 1;
var a = 2; //  OK

let b = 1;
let b = 2; //  Error: Identifier 'b' has already been declared

4. Hoisting

All three are "hoisted" (moved to top), but:

var: Initialized as undefined → you can access before declaration (gets undefined)
let & const: Not initialized → accessing before declaration throws a ReferenceError (they're in a "temporal dead zone")

console.log(a); // undefined
var a = 5;

console.log(b); //  ReferenceError
let b = 5;

Primitive types

In JavaScript, primitive data types are the simplest kinds of values. They are not objects, and they are immutable, meaning their value cannot be changed directly

JavaScript has 7 primitive data types: string, number, boolean, null, undefined, bigint, and symbol

String: text
Number: numbers
Boolean: true/false
Null: nothing on purpose(intentional empty value)
Undefined: not yet given a value(value not assigned yet)
BigInt: very big number
Symbol: unique id

let name = "Raja";
let age = 27;
let isStudent = true;
let result = null;
let score;
let big = 12345678901234567890n;
let id = Symbol("id");

One important point: primitive values are copied by value, so if you assign one primitive variable to another, the new variable gets its own copy

== and === are different in JavaScript

== checks value only after converting types if needed
=== checks both value and type without converting types

5 == "5"    // true
5 === "5"   // false

0 == false  // true
0 === false  // false

null == undefined   // true
null === undefined  // false

JavaScript Object

Raja B — Thu, 25 Jun 2026 16:54:14 +0000

What is an Object?

A JavaScript object is a collection of properties (key-value pairs) that stores related data and functions

Simple: Like a real object (e.g., a car)
has properties (color, model, speed)
and
actions (drive, stop)

Basic Structure:

const person = {
    name: "Raja",      
    age: 27,           
    city: "Madurai"    
};

Syntax breakdown:


- {} = curly braces (object literal)

- key: value = property (key-value pair)

- : = separates key and value

- , = separates properties

Key Concepts

1. Properties (Key-Value Pairs)

Term	Meaning
Key	Property name (e.g., name, age)
Value	Property data (e.g., "Raja", 27)
Property	Full key-value pair together

Example:

const student = {
    name: "Bala",    // name = key, "Bala" = value
    roll: 101,       // roll = key, 101 = value
    passed: true     // passed = key, true = value
};

Values can be ANY data type:

const data = {
    string: "hello",      // string
    number: 42,           // number
    boolean: true,        // boolean
    null: null,           // null
    array: [1, 2, 3],     // array
    object: { x: 10 },    // another object
    function: function() { console.log("hi"); }  // function
};

2. Methods (Functions as Properties)

When a property value is a function, it's called a method

Example:

Creating Objects (6 Ways)

1. Object Literal (Best/Most Common)

const person = {
    name: "Raja",
    age: 20
};

Best for: Readability, simplicity, speed

2. Using new Object()

const person = new Object();
person.name = "Raja";
person.age = 20;

3. Object Constructor

function Person(name, age) {
    this.name = name;
    this.age = age;
}

const p1 = new Person("Raja", 20);

4. Using Object.create()

const personTemplate = {
    greet() { console.log("Hello!"); }
};

const person = Object.create(personTemplate);
person.name = "Raja";

5. Using Object.assign()

const person = Object.assign(
    {}, 
    { name: "Raja" }, 
    { age: 20 }
);

6. Using Object.fromEntries()

const person = Object.fromEntries([
    ["name", "Raja"],
    ["age", 20]
]);

Reference:

The History Of Programming

Raja B — Thu, 25 Jun 2026 16:52:32 +0000

1. Before Computers

Humans wanted machines to perform calculations automatically

1642 – Blaise Pascal built a mechanical calculator called the Pascaline
1673 – Gottfried Wilhelm Leibniz improved mechanical calculation machines

2. Programmable Machine Idea

1804 – Joseph Marie Jacquard created the Jacquard Loom
It used punched cards to control weaving patterns
This introduced the idea that a machine could follow instructions

3. Early Computer Concept

1830s – Charles Babbage designed the Analytical Engine
It was a general-purpose mechanical computer concept
It had memory, processing, and input/output ideas similar to modern computers

4. First Programmer

1843 – Ada Lovelace wrote detailed instructions (algorithms) for the Analytical Engine
She realized a machine could do more than arithmetic
Because of her notes, she is often called the first programmer

5. Foundations of Computer Science

1936 – Alan Turing proposed the Turing Machine
He showed how a machine could follow instructions to solve problems
This became a foundation of modern computing

6. First Modern Programming Languages

1957 – John Backus created FORTRAN
One of the first successful high-level programming languages

7. Popular Languages

1972 – Dennis Ritchie created C
1985 – Bjarne Stroustrup created C++
1991 – Guido van Rossum created Python
1995 – James Gosling created Java
1995 – Brendan Eich created JavaScript

Simple Chain

Mathematics & Logic
⬇️
Jacquard Loom (Punch Cards)
⬇️
Charles Babbage (Computer Concept)
⬇️
Ada Lovelace (First Algorithm for a Computer)
⬇️
Alan Turing (Theory of Computation)
⬇️
FORTRAN, C, C++, Java, Python, JavaScript
⬇️
Modern Software and Apps

My opinion

ChatGPT gives answers when questions are asked. But sometimes the origin of an answer may not be known.

We can think of three possibilities:

Nobody knows who originally created or discovered it
Someone may have intentionally created it and wanted others to know about it
Someone may have wanted to hide or conceal its true origin

"கேள்வி கேட்க பழகு" → "Get used to asking questions."

"வரலாறு முக்கியம் அமைச்சரே!" "History matters, Minister!"

JavaScript Synchronous and Asynchronous

Raja B — Tue, 16 Jun 2026 15:31:06 +0000

Definition:

Type	Simple Meaning
Synchronous	Code runs one after another - each task waits for previous to finish
Asynchronous	Code runs independently - tasks don't block other code while waiting

Synchronous: "Wait for me to finish before you start" [blocking]

Asynchronous: "You start while I wait - call me back when done" [non-blocking]

1. Synchronous JavaScript (Normal/Default)

How it works:

Code executes line by line in order
Each statement waits for previous one to complete
Blocks execution until task finishes

Example:

Characteristics:

Feature	Description
Execution	Fixed sequence
Blocking	Yes - blocks other code
Complexity	Simple, easy to understand
Speed	Slower for time-consuming tasks
Best for	Simple, sequential tasks

2. Asynchronous JavaScript (Modern)

How it works:

Task starts in background
Code continues without waiting
Gets callback when task completes

Example (Non-blocking):

Characteristics:

Feature	Description
Execution	Tasks run independently
Blocking	No - non-blocking
Complexity	More complex (callbacks, promises)
Speed	Faster for slow operations
Best for	API calls, timers, file operations

Asynchronous Patterns in JavaScript

1. Callbacks (Old way)

Callback: Function that runs later - "calls you back" when done

2. Promises (Modern way)

Promise: Object representing async operation completion

States: pending → fulfilled or rejected

3. async/await (Best way - clean)

async: Function returns Promise
await: Waits for Promise (but doesn't block!)

Important Points

JavaScript is Synchronous by Default

2.JavaScript is Single-Threaded

Only 1 Call Stack

Executes one thing at a time

3.But Async Makes It Concurrent

Event Loop + Web APIs enable multiple tasks

4.Common Async Operations

Operation	Async Method
Timer	setTimeout()
API Call	fetch()
File Read	readFile()
Events	addEventListener()

Summary

Question	Answer
What is sync?	Code runs one after another
What is async?	Code runs independently, non-blocking
Which blocks?	Sync blocks, async doesn't
Default in JS?	Synchronous
How async works?	Event Loop + Web APIs + Callback Queue
Best async pattern?	async/await (clean code)

Reference:

JavaScript Scope: Block Scope, Global Scope & Function Scope

Raja B — Tue, 16 Jun 2026 14:32:06 +0000

Scope = where a variable is accessible (where you can use it)

1.Global Scope

Definition:

Variables declared outside any function or block have global scope

Example:

Problem: Global variables can be changed from anywhere → bugs!

2.Function Scope (Local Scope)

Definition:

Variables declared inside a function have function scope. They are only accessible inside that function

Example:

3.Block Scope(ES6 - Modern JavaScript)

Definition:

Variables declared with let or const inside a block {} have block scope. They are only accessible inside that block

What is a Block?

Any code inside curly braces {}:

if statements
for loops
while loops
Standalone blocks {}

Example:

Key Takeaways:

Global = accessible everywhere (declare outside functions)
Function = only inside function (declare inside function() {})
Block = only inside {} (declare with let/const inside blocks)
var = function scope (NOT block scope)

5.let/const = block scope (modern, preferred)

Process: A running program that has its own memory and resources — like the WhatsApp application

Thread: A smaller part inside a process that shares the same memory — like WhatsApp downloading messages, playing videos, and sending photos at the same time

Thread = Lightweight process

Uses shared memory
Faster to create and manage than a process
Helps perform multiple tasks concurrently

Simple Example:

Process: WhatsApp

Threads inside WhatsApp:

Message receiving thread
Photo sending thread
Video playing thread
Notification thread

One process can contain multiple threads

Reference:

ILUGC meetup (IIT Madras)

Raja B — Sun, 14 Jun 2026 10:20:04 +0000

ILUGC Meetup:

(ILUGC = Indian Linux Users Group Chennai)

I visted IIT Madras for the first time,and i was very excited. The environment there was amazing
The main reason i went there was because of the ILUGC meetup.A mentor suggested that i should try attending once,so i decided to participate
The founder spoke at the event.Since it was my first time attending,I could not fully understand everything that was discussed.However,the founder mentioned that things would become clearer if i continued attending regularly
Everyone introduced themselves,which helped me meet many people.They shared their opinion and experiences about linux and windows.Most of the discussions focused on linux and its benefits.
Some participants came with their families,which was nice to see.One person said that they started lerning linux when they were in 11th standard
Another participants explained web design, creative writing,and the benefits know of technology.There was also a discussion about creating more AI tool and resources in Tamil
One parent mentioned that children should also be introduced to linux so that they can improve their knowledge from a young age
Everyone was given an opportunity to speak,which was one of the best parts of the meetup.Peope shared their own knowledge and experiences,and question from participants were answered openly
Overall,attending the ILLUGU meetup was a valuable experience for me.Even though i did not understand everything,i learned a little more about linus and open source software.I hope to attend future meetups and continue leatning

Microsoft's current CEO, Satya Nadella, 
officially announced that 
'Microsoft Loves Linux'

Microsoft has provided a feature called 
WSL (Windows Subsystem for Linux) to run the Linux OS 
directly inside Windows 10 and 11. 
Furthermore, Microsoft has turned into one of the 
top financial contributors 
to the development of Linux

In 2026, Microsoft is not just a Windows company. 
Driven by the necessities of Cloud and AI technologies, 
it has emerged today as one of the most important developers 
and partners of Linux!

JavaScript Naming Conventions

Raja B — Fri, 12 Jun 2026 14:31:23 +0000

Programming has many naming conventions used to write clean and readable code

Style	Example
camelCase	`firstName`
PascalCase	`FirstName`
snake_case	`first_name`
UPPER_SNAKE_CASE	`MAX_SIZE`
kebab-case	`first-name`
Train-Case	`First-Name`
flatcase	`firstname`
UPPERCASE	`FIRSTNAME`
camel_Snake_Case	`first_Name`

Most Common Styles:

1. camelCase:

firstName mobileNumber

Used for:

Variables
Functions

2. PascalCase:

FirstName StudentDetails

Used for:

Classes
Components

3. snake_case:

first_name mobile_number

Common in:

Python
Databases

4. UPPER_SNAKE_CASE:

MAX_SIZE PI_VALUE

Used for:

Constants

5. kebab-case:

first-name user-profile

Used for:

URLs
CSS class names

In JavaScript, you cannot use first-name as a variable name because the-(hyphen) is interpreted as the minus operator.

 Error:

let first-name = "Raja";

Rule: Variable names can contain letters, numbers, _ (underscore), and $, but not - (hyphen)

Function

Raja B — Fri, 12 Jun 2026 14:30:40 +0000

Function:

A Function is a block of code written to perform a specific task.
Instead of writing the same code multiple times, we can place it inside a Function and call it whenever needed.

1. Parameters (In Function Definition):

Parameters are the variables written when creating a function

function biriyani(rice, masala, chicken) {
    console.log(rice, masala, chicken);
}

Here:

rice, masala, chicken

These are called Parameters

2. Arguments (In Function Call):

Arguments are the actual values passed when calling a function

biriyani("Basmati Rice", "Spicy Masala", "Chicken");

Here:

"Basmati Rice"
"Spicy Masala"
"Chicken"

These are called Arguments

    function eat(food) {   // food = Parameter
    console.log(food);
}

    eat("Biriyani");       // "Biriyani" = Argument

Parameter = In the function definition

Argument = In the function call

Return:

returnis used to send a value out from a function or return sends a value back from a function

       function biriyani(riceContainer, container, masalaContainer)
        {
            console.log(riceContainer, container, masalaContainer);
            console.log("biriyani ready");
            return "ChickenBiriyani";
        }

        function eat(food) 
       {
            console.log(food);
        }

        biriyani('rice', 'masala', 'chicken');
        eat();

Output:

1. Function With Arguments:

biriyani('rice', 'masala', 'chicken');

riceContainer, container, masalaContainer → Parameters
'rice', 'masala', 'chicken' → Arguments

The biriyani() function receives values when it is called, so it is a Function With Arguments

2. Function With Return Value:

return "ChickenBiriyani";

The biriyani() function returns a value ("ChickenBiriyani"), so it is a Function With Return Value

3. Function Without Arguments:

eat();

The eat() function is called without passing any value, so it is a Function Without Arguments

However:

function eat(food) {
    console.log(food);
}

This function has a parameter (food), but you called it without an argument. Therefore food becomes undefined

Hoisting in JavaScript:

Hoisting means JavaScript moves declarations to the top of their scope before the code runs

console.log(name);

var name = "Raja";

Output:

JavaScript treats it like:

var name;

console.log(name);

name = "Raja";

Output:

Function Hoisting:

sayHello();

function sayHello() {
    console.log("Hello");
}

Output:

Because function declarations are hoisted completely

let and const:

console.log(age);

let age = 25;

Output:

Coding Program js

Raja B — Thu, 11 Jun 2026 14:54:33 +0000

5.Multiples of 3 and 5 js program:

            for (let i = 1; i <= 100; i++) 
           {

             if (i % 3 === 0 && i % 5 === 0) 

            {
                console.log(i);
            }
        }

This use for program == Compares value only (type conversion happens) == loose equality
This use for program === Compares value and data type (no type conversion). === strict equality

Output:

6.Multiples of 3 or 5:

            for (let i = 1; i <= 30; i++) {

            if (i % 3 === 0 || i % 5 === 0) {

                console.log(i);
            }
        }

Output:

7.Divisors of given number:

        let num = 12;

        for (let i = 1; i <= num; i++) {

            if (num % i === 0) {

                console.log(i);
            }
        }

Output:

Questions & Answer

Raja B — Wed, 10 Jun 2026 10:27:08 +0000

(Since the if condition is false, "hello" is skipped; and because there are no curly braces {}, "bye" runs normally as an independent line.)

(Since the if condition is true, this line runs "hello" ; and because there are no curly braces {}, "bye" runs normally as an independent line.)

Since i stays 5 and never changes, the condition i >= 1 is always true, causing an infinite loop that prints "hello" forever without reaching "bye"

This use for infinite loop but methods overflow next line Bye not excute output but not space

Loop for and Do-while & Funtion

Raja B — Wed, 10 Jun 2026 10:22:50 +0000

For Loop:

A for loop is used when you know how many times you want to repeat a block of code

No Output:

This use for condition false but Ans: No output

Loop Check:

Entire query is about loop entry check and exit check examples in real-life situations.

Entry Check Example (Real-Life):

Example: Taking a Bus

Real-life situation:

Before entering the bus, you check: "Do I have enough money?" (condition checked first)
If the money you have is less than the bus ticket price, you never enter the bus (loop doesn't run)
You check the condition before taking the ride

Code example:

let count = 0;
while (count < 3) {
  console.log("Count: " + count);
  count++;
}

// Output: 0, 1, 2 (checks before each time)

Exit Check Example (Real-Life) Do-While Loop:

Example: ATM Machine

Real-life situation:

You first use the ATM (withdraw money)
Then you ask: "Do I want to withdraw more?" (condition checked after)
You always use the ATM at least once, even if you don't want more

Code example:

do {
  console.log("Enter amount");
  console.log("Withdraw money");
} while (wantMoreWithdrawal);

Function:

A Function is a block of code written to perform a specific task.(ஒரு குறிப்பிட்ட வேலை (task) செய்ய எழுதப்படும் code block-ஐ Function என்பார்கள்.)
Instead of writing the same code multiple times, we can place it inside a Function and call it whenever needed.(ஒரே code-ஐ பல முறை எழுதாமல், Function-ல் வைத்து தேவையான போது அழைக்கலாம் (call).

Syntax:

function functionName() 
{
    // code
}

Why do we use Functions?

Code Reuse – The same code can be used multiple times
Easy Maintenance – It is easier to modify and manage the code
Reduce Duplicate Code – No need to write the same code repeatedly
Better Readability – Makes the code easier to read and understand

Types of Functions in JavaScript:TBD

Normal Function:

function test() 
{
    console.log("Normal Function");
}

2.Function Expression:

const test = function() 
{
    console.log("Function Expression");
};

3.Arrow Function:

const test = () => 
{
    console.log("Arrow Function");
};

Reference:

Loop for While

Raja B — Tue, 09 Jun 2026 08:22:41 +0000

while loop:

When the number of iterations is unknown, but the condition is known

A while loop is used when you do not know 
how many times a task will be repeated, 
but you know the condition 
that must be met before the loop stops.

Real-life Example: Charging a mobile phone. The phone continues charging until the battery reaches 100%. It may take 20 minutes or 50 minutes (the exact number of repetitions is unknown), but the charging process continues as long as the battery is not fully charged.

Input:

        let battery = 90; 

        while (battery < 100) 
       {
            battery++; 
      console.log("Charging... Current level: " + battery);

        }

    console.log("Battery is fully charged! 
Please unplug the charger.");

Output:

Quick Memory Trick (Cheat Sheet):

Ascending Order (< or <=):

When you want to go from a smaller number to a larger number,
say:

"Run as long as our counter i is less than the larger number."
(Usually, we use i++ to increase the counter.)

Descending Order (> or >=):

When you want to go from a larger number to a smaller number, say:

"Run as long as our counter i is greater than the smaller number."
(Usually, we use i-- to decrease the counter.)

Input:

        let i = 1;

        while (i <= 5) {
            console.log(i);
            i++; 
        }

Output:

Input:

        let i = 5;

        while (i >= 1) {
            console.log(i);
            i--; 
        }

Output:

DEV Community: Raja B

Interview Question

**Primitive types**

== and === are different in JavaScript

JavaScript Object

Reference:

The History Of Programming

JavaScript Synchronous and Asynchronous

Asynchronous Patterns in JavaScript

Reference:

JavaScript Scope: Block Scope, Global Scope & Function Scope

Reference:

ILUGC meetup (IIT Madras)

ILUGC Meetup:

JavaScript Naming Conventions

Most Common Styles:

Function

Coding Program js

5.Multiples of 3 and 5 js program:

6.Multiples of 3 or 5:

7.Divisors of given number:

Questions & Answer

Loop for and Do-while & Funtion

Entry Check Example (Real-Life):

Exit Check Example (Real-Life) Do-While Loop:

Function:

Reference:

Loop for While

while loop:

Quick Memory Trick (Cheat Sheet):

Primitive types