DEV Community: Anees Abdul

Arrays in Javascript

Anees Abdul — Sat, 18 Apr 2026 15:29:39 +0000

What is an Array:

An array is a collection of values stored in a single variable. Instead of creating multiple variables:

let a = 10;
let b = 20;
let c = 30;

we can use Array:

let numbers=[10,20,30];

Creating an array using literal:

Creating Empty Array:
let a=[];
console.log(a);

Initalize values:
let b=[10,20,30];
console.log(b);

O/P:
[]
10,20,30

Creating using new Keyword(constructor):

let a= new Array(10,20,30);
console.log(a)
O/P:
[10,20,30]

1. Accessing Elements of an Array:
Any element in the array can be accessed using the index number. The index in the arrays starts with 0.

let fruits = ["apple", "banana", "mango"];

Using index:
console.log(fruits[0]);
O/P:
apple

First element:
let a=fruits[0];
console.log(a);
O/P:
apple

Last element:
let last=fruits[fruits.length-1];
console.log(last);
O/P:
mango

2. Modifying Array elements:

let fruits = ["apple", "banana", "mango"];
fruits[1] = "orange";
console.log(fruits);
O/P:
["apple", "orange", "mango"]

3. Adding Elements to the Array:

Elements can be added using push() and unshift() methods

push() → add at end
fruits.push("grape");
O/P:
"apple", "banana", "mango", "grape"

unshift() → add at beginning
fruits.unshift("kiwi");
"kiwi", "apple", "banana", "mango", "grape"

4. Removing Elements from an Array

To remove the elements from an array we have different methods like pop(), shift(), or splice().

pop() → remove last
fruits.pop();

shift() → remove first
fruits.shift();

5. Array Length:

let fruits = ["apple", "banana", "mango"];
console.log(fruits.length);

O/P:
3

6. Increase and Decrease the Array Length:

let fruits = ["apple", "orange", "kiwi"]
console.log(fruits.length);
fruits.length=7;
console.log(fruits);
O/P:
3
7 ['apple', 'orange', 'kiwi', empty × 4]

7. Iterating Through Array Elements
We can iterate array and access array elements using for loop and forEach loop.

let fruits = ["apple", "orange", "kiwi"]
for(let i=0;i<fruits.length;i++){
    console.log(fruits[i])
O/P:
apple
orange
kiwi

8. Array Concatenation

let fruits = ["apple", "orange", "kiwi"]
let basket=[1,2];
console.log(fruits.concat(basket));

O/P:
['apple', 'orange', 'kiwi', 1, 2]

9. Conversion of an Array to String
We have a builtin method toString() to converts an array to a string.

let fruits = ["apple", "orange", "kiwi"]
console.log(fruits.toString());
O/P:
apple,orange,kiwi

Objects in Javascript:

Anees Abdul — Fri, 17 Apr 2026 14:27:35 +0000

What is object:

An object is a collection of key-value pairs.
The values of properties can be primitives, objects, or functions (known as methods when defined inside an object).

let person = {
    name: "ABC",
    age: 30,
    city: "Chennai"
};

console.log(person);

{name: "ABC", age: 30, city: "Chennai"}

1. Accessing Object Properties
1️⃣ Dot Notation

console.log(person.name);

2️⃣ Bracket Notation

console.log(person["age"]);

2. Modifying Object Properties

It can be modified by reassigning its values.

let person= { name: "ABC", age: 30 };
console.log(person);

person.age=23//Modifying property
person.name="Test"

3. Adding Properties to an Object

You can dynamically add new properties to an object using dot or bracket notation.

person.country = "India";   // add

console.log(person);

4. Removing Properties from an Object
The delete operator removes properties from an object.

delete person.city;

console.log(person);

{name: "Anees", age: 31, country: "India"}

5. Checking if a Property Exists
You can check if an object has a property using the in operator

let person = { MobileNo: 1231231212 };
console.log("MobileNo" in person);

Adding objects with Methods
Objects can also store functions (called methods)

let person = {
    name: "And",
    greet: function() {
        console.log("Hello " + this.name);
    }
};

person.greet();

O/P:
Hello And

Looping through Object

let person = {
    name: "ABC",
    age: 30
};

for (let key in person) {
    console.log(key + ": " + person[key]);

O/P:
name: ABC
age: 30

Nested Objects
Objects inside objects

let user = {
    name: "ABC",
    address: {
        city: "Chennai",
        pincode: 600001
    }
};

console.log(user.address.city);

O/P:
Chennai

Array of Objects

let users = [
    { name: "ABC", age: 30 },//index 0
    { name: "John", age: 25 }//index 1
];
console.log(users[0].name);

O/P:
ABC

Callback function Examples

Anees Abdul — Thu, 16 Apr 2026 07:37:38 +0000

1. Greet

function welcome(name,cb){

    console.log("Hello: " +name);
    cb(name);

}
function sayBye(name){
    console.log("Bye:" +name);
}
welcome("Jo",sayBye);

Hello: Jo
Bye:Jo

2. Create Account to login

function createAccount(username,password,cb){
console.log("Provide details to login");
cb(username, password);

}
function login(username,password){

    console.log("User logged in:"+username, +password);
}
createAccount("Anees","123", login)

Provide details to login
User logged in:Anees 123

3. Clear interview to get job:

function clearInterview(cb){
    console.log("Selected");
     cb();

}
function gotJob(cb){
    console.log("Got placed");


}
clearInterview(gotJob);

Selected
Got placed

4. Even Number Check:

function check(a,cb){
console.log("Enter the number to check: " +a);
cb(a);
}
function evenNumberorNot(no){

console.log(no%2==0 ? "Even": "odd")

}
check(4,evenNumberorNot);

Enter the number to check: 4
Even

5. setTimeout Example

function fetchData(cb){
console.log("Fetching data");
//setTimeout(function,timeinMilliseconds);
setTimeout(()=>{
            let data={name:"User", age: "30"};
            cb(data);
            },2000);
}
function displayData(data){
    console.log("User:"+data.name, data.age);
}

fetchData(displayData);

Fetching data
User:User 30

Functions in JavaScript

Anees Abdul — Wed, 08 Apr 2026 14:07:24 +0000

What is a Function?
-A function is a block of code designed to perform a specific task.

Instead of writing the same code again and again, we can put it inside a function and reuse it.

function functionName() {
    // code to execute
}

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

greet(); // calling the function
O/P: Hello

Functions with Parameters

Parameters allow you to pass values into a function.

function greet(name) {
    console.log("Hello " + name);
}

greet("Anees");
O/P: Hello Anees

Functions with Return Value

A function can return a value using return.

function add(a, b) {
    return a + b;
}

let result = add(2, 3);
console.log(result);
O/P: 5

Types of Functions in JavaScript
1- Function Declaration
2- Function Expression
3- Arrow Functions (Modern JavaScript)
4- Anonymous Function
5- Callback Function
6- IIFE (Immediately Invoked Function)

What is Function Declaration:

A function declaration (or simply a function) is defined using:
The function keyword
function name
Parameters in parentheses
code block brackets

Hoisted (can be used before declaration)

function add(a, b) {
return a + b;}

function show() {
    console.log("Function Declaration");

What is Function Expression:
A function expression is a way of creating a function and assigning it to a variable.

A function expression is defined using:

The function keyword
Parameters in parentheses
A code block brackets

Not hoisted - Must be defined before calling.

const add = function(a, b) {return a + b;};

const add = function(a, b) {
    return a + b;
};

console.log(add(3, 4));

O/P:7

What is Arrow Function:

Arrow Functions allow a shorter syntax for function expressions.

You can skip the function keyword, the return keyword, and the curly brackets:

An arrow function uses the => symbol.
An arrow function is always written as a function expression.

const multiply = (a, b) => a * b;
console.log(multiply(3,4));
O/P: 12

What is an Anonymous Function:

An anonymous function is a function without a name.
It is usually used where functions are needed temporarily. Often used in timers or callbacks.

setTimeout(function() {
    console.log("Hello after delay");
}, 1000);
O/P: Hello after delay

What is Callback Function:

A callback function is a function passed as an argument to another function. -It is executed after a specific task is completed. Commonly used in asynchronous operations.

function process(name, callback) {
    console.log("Processing " + name);
    callback();
}

function done() {
    console.log("Done!");
}

process("Anees", done);

O/P: 
Processing Anees
Done!

How it works

process is called
Prints "Processing Anees"
Calls callback()
done() runs

What is IIFE (Immediately Invoked Function):

An IIFE is a function that runs immediately after it is created. No need to call it separately.

(function() {
    console.log("IIFE executed");
})();
O/P: IIFE executed

What is Closure:

A closure is a function that remembers variables from its outer function. Even after the outer function finishes, the inner function can still access those variables. Used for data privacy and maintaining state.

Looping Statements in JavaScript!

Anees Abdul — Mon, 06 Apr 2026 14:25:38 +0000

What is a Loop?
A loop repeats a set of instructions until a condition is met.

Example:
Instead of writing console.log("Hello") 5 times, we can use a loop.

Types of Loops in JavaScript
JavaScript provides several types of loops:

for loop
while loop
do...while loop
for...of loop(To be discussed)
for...in loop

1. for loop:

for loop is used to execute a block of code repeatedly, until a specified condition evaluates to false.

for (initialization; condition; increment/decrement) {
    // code to execute
}

Best used when you know how many times to loop.

for (let i = 1; i <= 5; i++) {
    console.log("Hello " + i);
}
O/P: 
Hello 1
Hello 2
Hello 3
Hello 4
Hello 5

2. while Loop:
The while loop runs as long as the condition is true.

while (condition) {
    // code
}

Used when the number of iterations is not fixed.

let i = 1;

while (i <= 5) {
    console.log(i);
    i++;
}
O/P:
1
2
3
4
5

3. do...while Loop:
This loop runs at least once, even if the condition is false.

do {
    // code
} while (condition);

Useful when the code must run at least once.

let i = 1;

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

4. for...of Loop:
Used to iterate over the values of arrays or strings.
Best for arrays and iterable objects.

let fruits = ["apple", "banana", "mango"];

for (let fruit of fruits) {
    console.log(fruit);
}
O/P:
apple
banana
mango

5. for...in Loop:
Used to iterate over keys (properties) of an object.
Best for objects.

let person = {
    name: "Anees",
    age: 30
};

for (let key in person) {
    console.log(key + ": " + person[key]);
}
O/P:
name: Anees
age: 30

Important Keywords:

break
continue

break: Stops the loop completely.

for (let i = 1; i <= 5; i++) {
    if (i === 3) break;
    console.log(i);
}
O/P:
1
2

continue: Skips the current iteration.

for (let i = 1; i <= 5; i++) {
    if (i === 3) continue;
    console.log(i);
}
O/P:
1
2
4
5

Advantages of Loops

Saves time and reduces code duplication
Makes code cleaner and readable
Handles repetitive tasks easily
Useful in arrays, objects, and automation

JavaScript program task-1

Anees Abdul — Mon, 06 Apr 2026 14:23:25 +0000

1) 1 1 1 1 1

let output = " ";
for(let i=1;i<=5;i++){
    output +=" 1";
    }
console.log(output);

1 1 1 1 1

2) 1 2 3 4 5

let output = " ";
for(let i=1;i<=5;i++){
    output += i + " ";
    }
console.log(output);

1 2 3 4 5

let output = " ";
for(let i=1;i<=10;i++){
    if(i%2!=0){
    output += i + " ";
    }
}
console.log(output);

 1 3 5 7 9

let output = " ";
for(let i=3;i<=15;i++){
    if(i%3==0){
    output += i + " ";
    }
}
console.log(output);

3 6 9 12 15

5) Multiples of 3 and 5

let output = " ";
for( let i=1;i<=50;i++){
    if((i%3==0)&&(i%5==0)){
        output +=i + " ";
    }
}
console.log(output);

 15 30 45

6) Multiples of 3 or 5

let output = " ";
for( let i=1;i<=50;i++){
    if((i%3==0)||(i%5==0)){
        output +=i + " ";
    }
}
console.log(output);

 3 5 6 9 10 12 15 18 20 21 24 25 27 30 33 35 36 39 40 42 45 48 50

7) Divisors of given number

let number=10;
for( let i=1; i<=20;i++){
    if(number%i==0){
        console.log(i);
    }
}

8) Count of Divisors of given number:

let number=10;
let count=0;
for( let i=1; i<=20;i++){
    if(number%i==0){
        //console.log(i);
        count++;

    }

}
 console.log(count);

9) Prime Number:

let number=9;
let count=0;
for( let i=1; i<=number;i++){
    if(number%i==0){
        count++;
    }
}
if(count==2){
    console.log("Is a Prime number");

}else{
    console.log("Not a Prime");
}

Not a Prime

10) Reverse a Number:

let number=1234;
let answer=0;

while(number>0){
    let digit=number%10;
    answer=answer*10+digit;
    number=Math.floor(number/10);
}
console.log(answer);

Task on Looping Statement

Anees Abdul — Fri, 03 Apr 2026 10:51:10 +0000

1) Print numbers from 1 to 5:

FlowChart:

let i=1;
   for(i=1;i<=5;i++){
    console.log(i);
   } 
O/P:
1
2
3
4
5

2) Program to print numbers 5 4 3 2 1
FlowChart:

for(let i=5;i>=1;i--){
    console.log(i);
}
O/P: 
5
4
3
2
1

3) Print even numbers from 1 to 10
FlowChart:


for(let i=1;i<=10;i++){
    if(i%2==0){
        console.log(i);
    }
}
O/P:
2
4
6
8
10

Conditional Statements in JavaScript:

Anees Abdul — Thu, 02 Apr 2026 11:50:30 +0000

What are Conditional Statements in JavaScript?

Conditional statements are used to make decisions in a program.

Conditional statements include:

if
if...else
if...else if...else
switch
ternary (? :)

1. if Statement:

Executes code only if the condition is true

let age = 20;

if (age >= 18) {
  console.log("You are an adult");
}

2. else Statement:

Use else to specify a code block to be executed if the same condition is false.

if (condition) {
  // code to execute if the condition is true
} else {
  // code to execute if the condition is false
}

3. else if Statement:

Used to check multiple conditions

let marks = 75;

if (marks > 90) {
  console.log("Excellent");
} else if (marks > 60) {
  console.log("Good");
} else {
  console.log("Average");
}

4. switch Statement:

Used to compare one value with multiple cases

let day = 2;

switch (day) {
  case 1:
    console.log("Monday");
    break;
  case 2:
    console.log("Tuesday");
    break;
  default:
    console.log("Invalid");
}

Ternary Operator:

Short form of if-else

condition ? expression1 : expression2


If the value of age is < 18, 
set the value of text to "Minor", otherwise to "Adult":

let text = (age < 18) ? "Minor" : "Adult";

Important Concepts of javascript is: Truthy Falsy:

In JavaScript, every value is treated as either true or false when used inside a condition.

This is known as truthy and falsy values

Truthy → behaves like true
Falsy → behaves like false

Falsy Values:
Falsy values are values that are considered false in a boolean context.

List of Falsy Values:

false
0
""
null
undefined
NaN

Example of Falsy:

let value = 0;

if (value) {
  console.log("Truthy");
} else {
  console.log("Falsy");
}

O/P: Falsy

Truthy Values

All values except falsy values are truthy.

Why It is Important:

*Reduces code length (no need for multiple conditions)
*Reduces unnecessary comparisons
*Useful for checking if a value exists
*Widely used in real-time applications (forms, APIs, login)

JavaScript Scenario Task:

Anees Abdul — Wed, 01 Apr 2026 16:47:53 +0000

1)Shopping Discount System:
In online shopping websites, discounts are applied based on the purchase amount.

Rules
Amount ≥ 5000 → 20% discount
Amount ≥ 2000 → 10% discount
Otherwise → No discount
Members get an extra 10% discount

   let Amount=5000;
   let isMember=true;
   let discount=0;
   if(Amount>=5000){
    discount=0.20;
   }else if(Amount>=2000){
    discount=.10;
   }
   if(isMember){
    discount=discount+.10;
   }
    let finalPrice=Amount-(Amount*discount);


   console.log("The final Price is "+finalPrice);

O/P: The final Price is 3500

2) Login Access Check:

Username = "admin"
Password = "1234"
User is not blocked

let username="admin";
   let password="1234";
   let isBlocked=true;
      if(username=="admin"){
        if(password=="1234"){
          if(!isBlocked){
            console.log("User verified");
          }else{
            console.log("User Blocked");
          }
        }else{
          console.log("Not a valid password")
        }
      }else{
        console.log("Not a valid username")
      }
O/P: User Blocked

3)Traffic Signal:

Red → Stop
Yellow → Get Ready
Green → Go

    let red = "stop";
    let yellow = "get ready";
    let green = "go";
    let colour = "yellow";
    switch (colour) {
     case "red":
     console.log("stop");
     break;
     case "yellow":
     console.log("get ready");
      break;
     case "green":
     console.log("go");
      break;
     default:
      console.log("Enter the Proper signal colour")

}
O/P: get ready

    let red = "stop";
    let yellow = "get ready";
    let green = "go";
    let colour = "yellow";
if(colour=="red"){
  console.log("stop");
}else if(colour=="yellow"){
  console.log("get ready")
}else if(colour=="green"){
  console.log("go");
}else{
  console.log("Enter the proper colour")
}
O/P: get ready

4) ATM Withdrawal:

If amount > balance → Insufficient funds
If amount ≤ 0 → Invalid amount
Otherwise deduct amount

 let UserBalance=5000;
    let WithdrawAmount=1000;
    let FinalAmount=0;
    if(UserBalance>=WithdrawAmount){
        FinalAmount=UserBalance-WithdrawAmount;
      console.log("Amount withdrawn is "+WithdrawAmount +" and the Final Amount is " +FinalAmount);
    }else if(UserBalance<WithdrawAmount){
      console.log("Insufficient Fund")
    }
O/P: Amount withdrawn is 1000 and the Final Amount is 4000

5) Mobile Data Usage
Internet data usage.

Usage ≥ 100 → Limit exceeded
Usage ≥ 80 → Warning
Otherwise normal usage

let usage=40;
  if(usage>=100){
    console.log("Limit Exceeded");
   }else if(usage>=80){
    console.log("Warning: Usage is nearing limit");
   }else {
    console.log("Normal Usage");
   }
O/P: Normal Usage

6) Electricity Bill:
This calculates the bill amount based on units consumed.

0–100 → ₹5/unit
101–300 → ₹7/unit
Above 300 → ₹10/unit

let unitsConsumed=60;
   let bill=0;
   if(unitsConsumed<=100){
    bill=unitsConsumed*5;
   }else if(unitsConsumed>100 && unitsConsumed<300){
    if(unitsConsumed)
    bill=unitsConsumed*7;
   }else{
   bill=unitsConsumed*10;
   }
  console.log("Total Bill:"+bill);

O/P: Total Bill:300

Operators & Expressions in JavaScript

Anees Abdul — Mon, 30 Mar 2026 14:10:54 +0000

What are Operators in JavaScript?

Operators are symbols used to perform operations on values and variables.

Types of Operators in JavaScript
1. Arithmetic Operators:

Used to perform mathematical calculations
Operators List:

+ → Addition
- → Subtraction
* → Multiplication
/ → Division
% → Modulus

let a = 10;
let b = 5;

console.log(a + b); // 15
console.log(a - b); // 5
console.log(a * b); // 50
console.log(a / b); // 2
console.log(a % b); // 0

2. Assignment Operators:

Used to assign values to variables

let x = 10;

x += 5;  // x = x + 5 → 15
x -= 2;  // x = x - 2 → 13

=
+=
-=
*=
/=

3. Comparison Operators:

Used to compare two values (returns true/false)

console.log(10 > 5);   // true
console.log(10 == "10"); // true
console.log(10 === "10"); // false

== → Equal (value only)
=== → Strict Equal (value + type)
!= → Not equal
> < >= <=

4. Logical Operators:

Used to combine conditions

let age = 25;

console.log(age > 18 && age < 30); // true
console.log(age > 18 || age < 20); // true
console.log(!(age > 18));          // false

&& → AND
|| → OR
! → NOT

5. Unary Operators:
Operate on one value

let a = 5;

a++; // increment → 6
a--; // decrement → 5

Type Operators (Need to be discussed)

Used to check data types

let name = "Anees";

console.log(typeof name); // string

Type Coercion:
JavaScript sometimes automatically converts types

console.log(10 - "2"); // 8
console.log(10 + "2"); // "102"

Introduction to JavaScript!!

Anees Abdul — Sat, 28 Mar 2026 13:02:07 +0000

What is JavaScript:

It was created in 1995 by Brendan Eich in just 10 days.
You can run JavaScript directly in your browser — no installation needed to get started
JavaScript is used to create interactive and dynamic web pages. It is responsible for adding functionality to the website and allows use interaction with the website content.

Where to Write JavaScript?
You can write JavaScript in 3 ways:

Inline JavaScript:

<button onclick="alert('Hello')">Click Me</button>

Internal JavaScript:

<script>
  alert("Hello World");
</script>

External JavaScript (Best Practice):

<script src="script.js"></script>

Basic JavaScript Concepts

Variables:

Variables are containers for storing data. Variables can be declared in 3 ways:
Using var
Using let
Using const

Rules to name a variable:

No javascript keyword is allowed
Should not start with number

- No space in between a variable name and no -

Using var:

It is a function-scope or global scope (not block-scope).
It can be accessed inside and outside the block

In modern JavaScript, you declare them using let or const.

let — for values that change

It should have a Block Scope(local Scope) and must be declared before use. It can't be accessed outside the block.
Also, Variables declared with let cannot be redeclared in the same scope and can be reassigned.

let score = 0;
score = 10;  // ✅ we can reassign let
console.log(score); // 10

const — for values that never change:

It should have a block scope.
Variables defined with const cannot be redeclared.
Variables defined with const cannot be reassigned.

const PI = 3.14159;
// PI = 3; ❌ TypeError: Assignment to constant variable
console.log(PI); // 3.14159

Rule of thumb: Default to const. Only switch to let when you know the value will need to change. Avoid var — it's the old way and has quirky scoping rules.

Data Types of JavaScript:

Primitive DataTypes
Non - Primitive DataTypes

1.1Primitive Datatypes:

A JavaScript variable can hold 7 types of data:

Strings - A text of characters enclosed in quotes

let name = "Anees";

Numbers - A number representing a mathematical value

let age = 25;

Bigint - A number representing a large integer

let bigNumber = 12345678901234567890n;

Boolean - A data type representing true or false

let isActive = true;

undefined - A primitive variable with no assigned value

let x;

Null - A primitive value representing object absence

let y = null;

Symbol - A unique and primitive identifier

let id = Symbol("id");

2.1 Non- Primitive DataType:

Object - A collection of key-value pairs used to store structured data.

let person = {
  name: "Anees",
  age: 30
};

Array- An ordered list of values used to store multiple items in a single variable.

let numbers = [1, 2, 3];

Function - A reusable block of code designed to perform a specific task.

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

String Buffer/String Builder in Java

Anees Abdul — Tue, 24 Mar 2026 07:23:38 +0000

What is String Buffer:

StringBuffer is a mutable and thread-safe class used to modify strings.
String Buffer is a child of the CharSequence interface
StringBuffer is a **mutable class **in Java. When we perform operations like append or modify, it updates the same object instead of creating a new one, which improves performance.
StringBuffer is synchronised/THREAD-SAFE (Multiple threads can't access it simultaneously)

How to create a StringBuffer Object:

By using only new Keyword

StringBuffer s= new StringBuffer("Java");

Object will be created in heap memory, and the reference variable is pointing to it.

StringBuffer Methods:

append() → Add data Adds text at the end

StringBuffer sb = new StringBuffer("Java");
sb.append(" World");

System.out.println(sb);
O/P:Java World

insert() → Insert at specific position Adds text at a given index

StringBuffer sb = new StringBuffer("Java");
sb.insert(2, "XX");

System.out.println(sb);
o/P: JaXXva

replace() → Replace part of string Replaces characters between indexes

StringBuffer sb = new StringBuffer("Java");
sb.replace(1, 3, "XX");

System.out.println(sb);
O/P: JXXa

delete() → Delete characters Removes characters between indexes

StringBuffer sb = new StringBuffer("Java");
sb.delete(1, 3);

System.out.println(sb);
O/P: Ja

reverse() → Reverse the string

StringBuffer sb = new StringBuffer("Java");
sb.reverse();

System.out.println(sb);
O/P: avaJ

capacity() → Check memory capacity Default capacity = 16 + string length

StringBuffer sb = new StringBuffer("Java");
System.out.println(sb.capacity());
O/P: 20

length() → Length of string

System.out.println(sb.length());

charAt() → Get character

System.out.println(sb.charAt(1));

setCharAt() → Change character

StringBuffer sb = new StringBuffer("Java");
sb.setCharAt(0, 'M');

System.out.println(sb);
O/P: Mava

ensureCapacity() → Increase capacity

sb.ensureCapacity(50);
Ensures buffer can hold at least 50 characters

What is StringBuilder:

String Builder is a child of the CharSequence interface
String Builder is a MUTABLE object
StringBuilder is NON-Synchronised/NOT THREAD-SAFE(Multiple threads can access)

Methods are the same as StringBuffer