DEV Community: Pranay Rebeyro

Scope of Variables in JavaScript

Pranay Rebeyro — Sat, 11 Jul 2026 16:27:27 +0000

Scope
A variable is not always available everywhere in your program. Some variables can be used anywhere, while others can only be used inside a function or a block.This is called Scope.

Types of Scope in JavaScript

Global Scope - A variable declared outside any function or block is called a Global Variable.It can be accessed from anywhere in the program.

Eg:
let name = "Pranay";

function greet() {
    console.log(name);
}

console.log(name);

greet(); // Output:
Pranay
Pranay

Function Scope - Variables declared inside a function can only be used inside that function.

Eg:
function student() {
    let age = 20;

    console.log(age);
}

student(); // Output: 20

// If you try to access age outside the function
function student() {
    let age = 20;
}

console.log(age); // Output: ReferenceError: age is not defined

// This happens because age exists only inside the function.

Block Scope - A block is anything inside curly braces { }.Variables declared using let and const inside a block can only be accessed within that block.

Eg:
if (true) {
    let city = "Chennai";

    console.log(city);
} // Output: Chennai

// Trying to access it outside the block
if (true) {
    let city = "Chennai";
}

console.log(city); // Output: ReferenceError: city is not defined

var - Function-scoped and not block-scoped.
let and const - Block-scoped

Variable Hoisting
Hoisting means JavaScript processes declarations first, before running the program.

Hoisting - var and function
Non-Hoisting - let and const

Functions in JavaScript

Pranay Rebeyro — Sat, 11 Jul 2026 16:12:16 +0000

Function
Function is a reusable block of code designed to perform a specific task

Eg:
function greet() {
    console.log("Hello, World!");
}

greet(); // Output: Hello, World!

Function with Parameters
Sometimes we want to pass values to a function. These values are called parameters.

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

greet("Pranay"); // Output: Hello Pranay

Function with Multiple Parameters
A function can have more than one parameter.

Eg:
function add(a, b) {
    console.log(a + b);
}

add(10, 20); // Output: 30

Function with Return Value
Sometimes we don't want to print the result directly. Instead, we want the function to send the result back.For that, we use the return keyword.

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

let result = add(10, 20);

console.log(result); // Output: 30

// The return statement ends the function and returns the value.

Function Expression
A function can also be stored inside a variable.

Eg:
const greet = function() {
    console.log("Hello JavaScript");
};

greet(); // Output: Hello JavaScript

Arrow Function
Arrow functions are a shorter way to write functions.

**Arrow Function**
Eg:
const greet = () => {
    console.log("Hello JavaScript");
};

greet(); //Output: Hello JavaScript

**Arrow Function with Parameters**
Eg:
const add = (a, b) => {
    return a + b;
};

console.log(add(10, 20)); // Output: 30

// If the function has only one line, we can write it even shorter.

Eg:
const add = (a, b) => a + b;

console.log(add(10, 20)); // Output: 30

Anonymous Function
A function without a name is called an Anonymous Function.

Eg:
const message = function() {
    console.log("Welcome");
};

message(); // Output: Welcome

// Here, the function has no name and is stored inside the variable message.

Loops in JavaScript

Pranay Rebeyro — Sat, 11 Jul 2026 15:55:46 +0000

Loop
A loop is used to execute a block of code repeatedly until a condition becomes false.

1. for loop - The for loop is the most commonly used loop. It is useful when you know how many times the loop should run.
2. while loop - The while loop executes as long as the condition is true.
3. do while loop - The do...while loop executes the code at least once, even if the condition is false.

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

**while loop**
let i = 0;
while (i < 10) {
  i++;
  if (i === 3) continue; // Skips printing 3
  if (i === 6) break;    // Stops the loop completely at 6
  console.log(i);
} // Output:
1
2
4
5
6

**do while loop**
let i = 1;

do {
    console.log(i);
    i++;
} while (i >= 5); // Output: 1

break - Stops the loop immediately.
continue - Skips the current iteration and moves to the next one.

Conditional Statements in JavaScript

Pranay Rebeyro — Sat, 11 Jul 2026 15:27:55 +0000

Conditional Statements
Conditional statements allow JavaScript to execute different blocks of code based on whether a condition is true or false.

if statement

if - The if statement executes a block of code only if the condition is true.

let age = 20;

if (age >= 18) {
    console.log("Eligible to vote");
} //Output: Eligible to vote

if else statement

if...else - Use if...else when you want one block of code to run if the condition is true and another block if it's false.

let age = 16;

if (age >= 18) {
    console.log("Eligible to vote");
} else {
    console.log("Not eligible to vote");
} // Output: Not eligible to vote

if elif statement

if...else if...else - Use this when you have multiple conditions to check.

let marks = 85;

if (marks >= 90) {
    console.log("Grade A");
} else if (marks >= 75) {
    console.log("Grade B");
} else if (marks >= 50) {
    console.log("Grade C");
} else {
    console.log("Fail");
} // Output: Grade B

switch statement

switch statement - The switch statement is used when you have many possible values for one variable.

let day = 3;

switch (day) {
    case 1:
        console.log("Monday");
        break;

    case 2:
        console.log("Tuesday");
        break;

    case 3:
        console.log("Wednesday");
        break;

    default:
        console.log("Invalid Day");
} // Output: Wednesday

// Important: The break statement stops the execution after the matching case.We must compulsory to use break statement because if you don't use break, JavaScript will continue executing the next cases even the output is correct.

Nested if statement

Nested if statement - You can also write an if statement inside another if.

let age = 20;
let hasLicense = true;

if (age >= 18) {
    if (hasLicense) {
        console.log("You can drive.");
    }
} // Output: You can drive.

Ternary Operator

Ternary Operator - An optimized one-line shorthand for standard if...else blocks

let isLoggedIn = true;
let systemMessage = isLoggedIn ? "Welcome back!" : "Please log in.";

console.log(systemMessage); // Outputs: Welcome back!

Variables and Operators in JavaScript

Pranay Rebeyro — Sat, 11 Jul 2026 15:09:37 +0000

Variables in JavaScript
variables are containers used to store data values that can be manipulated or referenced later in a program

let - A variable declared with let can be changed later.(Block-Scoped)
const - A variable declared with const cannot be reassigned(Block-Scoped).
var - var was the first way to declare variables in JavaScript. It was used before let and const were introduced.One thing to remember is that var can also be redeclared(Global-Scoped).

Operators
Operators are special symbols or keywords used to perform calculations, compare values, or manipulate data.

1.Arithmetic Operators
Used to perform standard mathematical equations.

Addition(+): Adds numbers or links strings together.
Subtraction(-): Finds the difference between numbers. Multiplication(): Multiplies numbers.
Division(/): Divides Numbers to find the quotient.
Modulo(%): Returns the leftover division remainder.
Exponentiation(*): Raises a number to a power.

2.Assignment Operators
Used to store or update values inside variables.

Assignment(=): Assigns a value to a variable.
Addition Assignment(+=): Adds a value to a variable and updates it.
Subtraction Assignment(-=): Subtracts a value and updates the variable.
Multiplication Assignment(*=): Multiplies a variable and updates it.
Division Assignment(/=): Divides a variable and updates it

3.Comparison Operators
These compare two values and always return a boolean true or false

Equality(==): Checks if values match, ignoring their data type.
Strict Equality(===): Checks if both the values and data types match.
Inequality(!=): Checks if values are not equal.
Strict Inequality(!==): Checks if values or types do not match. Greater Than(>): Returns true if the left value is bigger.
Less Than(<): Returns true if the left value is smaller.
Greater Than or Equal(>=): Checks if the left value is bigger or equal.
Less Than or Equal(<=): Checks if the left value is smaller or equal.

4.Logical Operators
Used to combine multiple conditions or reverse boolean values.

AND(&&): Returns true if both conditions are true.
OR(||): Returns true if at least one condition is true.
NOT(!): Reverses a boolean value (true to false)

5.Unary Operators
Operators that require only a single value to work.

Increment(++): Increases a numeric value by 1.
Two types of Increment PostIncrement(i++) and PreIncrement(--i)
Decrement(--): Decreases a numeric value by 1.
Two types of Decrement PostDecrement(i++) and PreDecrement(--i)

Important

In addition operator only concatenation will happen when merge two different data types and in other operators we will try to merge different data types it will show "NaN"(Not-a-Number)

Addition
Eg: 10+"5" = 105;
"Sam"+2 = Sam2

Subtraction
Eg: 10-"2" = 8
"Sam"-2 = NaN // Here you can see a difference that in subtraction when try to merge the difference data types the outputs becomes Nan.This is same for other operators expect the addition operator.

**Increment**
PostIncrement:
let i = 5;
let result = i++;
console.log(result); // Output: 5 
console.log(i); // Output: 6

PreIncrement:
let i = 5;
let result = ++i; 

console.log(result); // Output: 6
console.log(i); // Output: 6

**Decrement**
PostDecrement:
let i = 5;
let result = i--;
console.log(result); // Output: 5 
console.log(i); // Output: 4

PreDecrement:
let i = 5;
let result = --i; 

console.log(result); // Output: 4
console.log(i); // Output: 4

JavaScript

Pranay Rebeyro — Sat, 11 Jul 2026 14:28:44 +0000

JavaScript is a lightweight, interpreted programming language used primarily to make web pages interactive and dynamic.JavaScript first name was "MOCHA".

DATA TYPES
JavaScript data types are divided into two categories.

Primitive Type - String,Number,Boolean,BigInt,Undefined,Null,Symbol
Non-Primitive Type - Arrays,Objects,Functions

Word - String
Number - number
Large no - BigInt
true/false - boolean

How to Check a Data Type?
JavaScript provides the typeof operator.


eg: let age = 25;

console.log(typeof age);

output: number

Static Typing
Languages like Java use Static Typing.

eg: int age = 25; // Here we must add which datatype the value  blongs to.

Dynamic Typing
Languages like JavaScript, Python use Static Typing.

`eg: age = 25; // Here we need not to mention the data type.`