💻 JavaScript Roadmap 2026 — Part 4

🟢 Conditional Statements & Loops

Programs often need to make decisions and repeat tasks.

For example:
• If a user is logged in → show the dashboard.
• If the payment fails → show an error.
• Repeat a task for every item in a list.
• Continue processing until a condition becomes false.

JavaScript provides conditional statements and loops for this.

1️⃣ "if" Statement

The "if" statement executes code only when a condition is "true".
let age = 20;

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

Since "age >= 18" is "true", the message is printed.

Basic structure
if (condition) {
// code to execute
}

2️⃣ "if...else"

Use "else" when you want to execute another block if the condition is false.

let age = 16;

if (age >= 18) {
console.log("You can vote");
} else {
console.log("You cannot vote");
}

Output: You cannot vote

3️⃣ "else if"

Sometimes there are multiple conditions.
let marks = 75;

if (marks >= 90) {
console.log("Grade A+");
} else if (marks >= 75) {
console.log("Grade A");
} else if (marks >= 60) {
console.log("Grade B");
} else {
console.log("Grade C");
}

Output: Grade A

JavaScript checks the conditions from top to bottom. Once it finds a true condition, the remaining "else if" blocks are skipped.

4️⃣ Multiple Conditions

You can combine conditions using logical operators.

let age = 25;
let hasID = true;

if (age >= 18 && hasID) {
console.log("Access granted");
}

Both conditions must be true because we used "&&".

Using OR
let hasEmail = false;
let hasPhone = true;

if (hasEmail || hasPhone) {
console.log("Contact information available");
}

At least one condition needs to be true.

5️⃣ Nested "if"

An "if" statement can exist inside another "if".

let isLoggedIn = true;
let isAdmin = true;

if (isLoggedIn) {
if (isAdmin) {
console.log("Admin dashboard");
}
}

Nested conditions can be useful, but too much nesting can make code difficult to read. When logic becomes complicated, consider restructuring the conditions.

🔹 "switch" Statement

6️⃣ What is "switch"?

"switch" is useful when you want to compare one value against several possible values.

let day = "Monday";

switch (day) {
case "Monday":
console.log("Start of the week");
break;
case "Friday":
console.log("Almost weekend");
break;
default:
console.log("Regular day");
}

Output: Start of the week

7️⃣ Why "break" Matters

let number = 2;

switch (number) {
case 1:
console.log("One");
break;
case 2:
console.log("Two");
break;
case 3:
console.log("Three");
break;
}

"break" tells JavaScript to exit the "switch". Without it, execution can continue into subsequent cases. This behavior is called fall-through.

8️⃣ "default"

"default" runs when none of the cases match.
let fruit = "Mango";

switch (fruit) {
case "Apple":
console.log("Apple");
break;
case "Banana":
console.log("Banana");
break;
default:
console.log("Unknown fruit");
}

Output: Unknown fruit

🔁 LOOPS

Loops allow you to execute code repeatedly.

Instead of writing:
console.log(1);
console.log(2);
console.log(3);

you can use a loop.

9️⃣ "for" Loop

The traditional "for" loop has three parts:

for (initialization; condition; update) {
// code
}
Example:
for (let i = 1; i <= 5; i++) {
console.log(i);
}

Output: 1 2 3 4 5

Let's break it down:
• let i = 1 → Start with 1.
• i <= 5 → Continue while "i" is less than or equal to 5.
• i++ → Increase "i" after every iteration.

🔟 Understanding an Iteration

For:

for (let i = 1; i <= 3; i++) {
console.log(i);
}

JavaScript roughly follows:

• i = 1 → condition true → print 1
• i = 2 → condition true → print 2
• i = 3 → condition true → print 3
• i = 4 → condition false → stop

Understanding this process is more important than memorizing the syntax.

1️⃣1️⃣ "while" Loop

A "while" loop runs as long as its condition remains true.

let count = 1;

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

Output: 1 2 3 4 5

Be careful to update the variable. This can create an infinite loop:

let count = 1;

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

"count" never changes, so the condition never becomes false.

1️⃣2️⃣ "do...while"

A "do...while" loop executes the code at least once, because the condition is checked after the code runs.

let count = 10;

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

Output: 10

Even though "count < 5" is false, the code runs once.

Compare:
while (count < 5) {
    console.log(count);
}

The "while" loop may execute zero times.

1️⃣3️⃣ "break"

"break" immediately stops a loop.

for (let i = 1; i <= 10; i++) {
    if (i === 5) {
        break;
    }
    console.log(i);
}

Output: 1 2 3 4

1️⃣4️⃣ "continue"

"continue" skips the current iteration and moves to the next one.

for (let i = 1; i <= 5; i++) {
    if (i === 3) {
        continue;
    }
    console.log(i);
}

Output: 1 2 4 5

1️⃣5️⃣ Nested Loops

You can put one loop inside another.

for (let i = 1; i <= 3; i++) {
    for (let j = 1; j <= 2; j++) {
        console.log(i, j);
    }
}

Output:
• 1 1
• 1 2
• 2 1
• 2 2
• 3 1
• 3 2

Nested loops are useful for:
• Grids
• Tables
• Matrices
• Combinations
• Certain algorithmic problems

But they can become expensive when working with large datasets.

⭐ 1️⃣6️⃣ "for...of"

"for...of" is especially useful for iterating over values in an iterable such as an array or string.

let fruits = ["Apple", "Banana", "Mango"];

for (let fruit of fruits) {
    console.log(fruit);
}

Output: Apple Banana Mango

⭐ 1️⃣7️⃣ "for...in"

"for...in" is commonly used to iterate over enumerable property keys of an object.

let person = {
    name: "Rahul",
    age: 25,
    city: "Pune"
};

for (let key in person) {
    console.log(key);
}

Output: name age city

To access the corresponding value:
for (let key in person) {
    console.log(key, person[key]);
}

Output: name Rahul, age 25, city Pune

Important distinction
• for...of → values
• for...in → property keys

🧠 Real-World Example

Imagine an e-commerce application. You want to check whether each customer has completed their payment.

let payments = [
    "success",
    "success",
    "failed",
    "success"
];

for (let payment of payments) {
    if (payment === "success") {
        console.log("Order confirmed");
    } else {
        console.log("Payment failed");
    }
}

Output: Order confirmed, Order confirmed, Payment failed, Order confirmed

💻 Practice

1️⃣ Voting eligibility
Write a program that prints Eligible when age is 18 or above, otherwise Not eligible

2️⃣ Even numbers
Use a loop to print: 2 4 6 8 10

3️⃣ Multiplication table
Print the multiplication table of 7

4️⃣ Find a number
Loop from 1 to 20 and stop when you reach 13. Use "break".

5️⃣ Skip numbers
Loop from 1 to 10 but skip 5. Use "continue".

6️⃣ Predict the output
for (let i = 1; i <= 5; i++) {
    if (i === 3) {
        continue;
    }
    console.log(i);
}
7️⃣ Challenge
Write a program that checks numbers from 1 to 20 and prints Even for even numbers and Odd for odd numbers.
Hint: number % 2 === 0

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💻 JavaScript Roadmap 2026 — Part 5

🟢 Functions in JavaScript

Functions are one of the most important concepts in JavaScript.

A function is a reusable block of code designed to perform a particular task. Instead of writing the same code repeatedly, you can put it inside a function and call it whenever you need it.

1️⃣ Creating a Function

The basic syntax is:

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

This creates a function called "greet". But creating a function doesn't execute it.

To execute it, you need to call it:

greet();

Output: Hello!

2️⃣ Why Do We Need Functions?

Imagine you need to print the same message three times.

Without a function:

console.log("Welcome to JavaScript");
console.log("Welcome to JavaScript");

With a function:

function welcome() {
    console.log("Welcome to JavaScript");
}

welcome();
welcome();
welcome();

Functions make code:

✅ Reusable,

✅ Easier to maintain,

✅ Easier to test,

✅ Easier to understand

3️⃣ Function Parameters

Functions can accept information through parameters.

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

greet("Rahul");
greet("Priya");

Output: Hello Rahul, Hello Priya

Here: name → parameter, "Rahul" → argument.

A parameter is the variable defined in the function. An argument is the actual value passed when calling it.

4️⃣ Multiple Parameters

A function can accept multiple parameters.

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

add(10, 5); // 15

You can reuse the same function: add(20, 30); add(100, 50);

5️⃣ Returning a Value

A function doesn't always need to print something. It can return a value.

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

let result = add(10, 5);
console.log(result); // 15

The important difference is: console.log() displays something. return sends a value back from the function.

6️⃣ Understanding "return"

function multiply(a, b) {
    return a * b;
}

let result = multiply(5, 4);
console.log(result);

The process is: multiply(5, 4) → 5 × 4 → 20 → return 20 → result = 20

Once JavaScript executes "return", the function stops executing.

function test() {
    return "Hello";
    console.log("This will not run");
}

7️⃣ Functions Without "return"

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

greet("Rahul");

That's completely valid. If a function doesn't explicitly return a value, its return value is: undefined

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

let result = greet();
console.log(result); // Hello, undefined

8️⃣ Storing a Function in a Variable

Functions can be assigned to variables.

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

greet();

This is called a function expression.

Compare:

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

Function expression: const greet = function () { console.log("Hello"); };

Both can be called as functions, but they have different hoisting behavior.

🔹 Arrow Functions

9️⃣ What Is an Arrow Function?

Arrow functions provide a shorter syntax for writing functions.

Traditional function:

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

Arrow function:

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

For a simple expression, you can make it even shorter:

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

This is called an implicit return.

🔟 Arrow Function Examples

One parameter:

const square = number => number * number;
console.log(square(5)); // 25

Multiple parameters:

const multiply = (a, b) => a * b;
console.log(multiply(4, 5)); // 20

No parameters:

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

1️⃣1️⃣ Default Parameters

You can provide a default value for a parameter.

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

greet("Rahul"); // Hello Rahul

greet(); // Hello Guest

Default parameters are very useful when a value is optional.

1️⃣2️⃣ Rest Parameters

Sometimes you don't know how many arguments will be passed.

You can use "...".

function add(...numbers) {
    console.log(numbers);
}

add(10, 20, 30, 40); // [10, 20, 30, 40]

The rest parameter collects the arguments into an array.

function add(...numbers) {
    let total = 0;
    for (let number of numbers) {
        total += number;
    }
    return total;
}

console.log(add(10, 20, 30)); // 60

1️⃣3️⃣ Scope

Scope determines where a variable can be accessed.

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

greet();

"message" exists inside the function. Trying to access it outside: console.log(message); will cause an error.

This is because "message" has function/local scope.

1️⃣4️⃣ Block Scope

Variables declared with "let" and "const" are block-scoped.

if (true) {
    let message = "Hello";
    console.log(message);
}

This works. But: console.log(message); outside the block does not. The block is defined by { // block }.

This is one reason "let" and "const" are preferred over "var".

1️⃣5️⃣ Global Scope

A variable declared outside functions or blocks can be accessible from broader parts of the program.

const appName = "My App";

function showAppName() {
    console.log(appName);
}

showAppName(); // My App

However, don't create unnecessary global variables. Too many globals can make larger applications difficult to maintain.

⭐ 1️⃣6️⃣ Callback Functions

A function can be passed to another function.

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

function processUser(callback) {
    callback("Rahul");
}

processUser(greet); // Hello Rahul

Here: greet is passed as a value to processUser. This is called a callback function. Callbacks are extremely important in JavaScript because you'll encounter them throughout: Events, Array methods, Asynchronous programming, APIs, Node.js

⭐ 1️⃣7️⃣ Higher-Order Functions

A higher-order function is a function that: accepts another function as an argument, or returns a function.

function calculate(a, b, operation) {
    return operation(a, b);
}

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

console.log(calculate(10, 5, add)); // 15

Here "calculate()" receives another function. That's a higher-order function.

⭐ 1️⃣8️⃣ Functions Returning Functions

A function can also return another function.

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

const greetRahul = createGreeting("Rahul");
greetRahul(); // Hello Rahul

This concept leads directly into one of JavaScript's most important advanced topics: Closures. We'll cover closures separately later.

1️⃣9️⃣ Pure Functions

A pure function generally: 1. Produces the same output for the same input. 2. Doesn't modify external state.

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

Every time: add(2, 3) returns: 5. A function that depends on changing external state can behave differently. Understanding pure functions becomes particularly useful when working with modern frontend frameworks.

🧠 Real-World Example

Imagine an online shopping application.

function calculateDiscount(price, discount) {
    return price - discount;
}

function calculateTax(price, taxRate) {
    return price * taxRate;
}

let price = 2000;
let discountedPrice = calculateDiscount(price, 300);
let tax = calculateTax(discountedPrice, 0.18);

console.log(discountedPrice);
console.log(tax);

Instead of putting everything into one huge block of code, each function handles a specific responsibility.

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