JavaScript Functions
Definition, Syntax, Scope, and Advanced Usage

Introduction to JavaScript Functions

In JavaScript, functions are reusable blocks of code designed to perform a specific task. They're the building blocks of any application – small or large. Functions help break complex problems into manageable parts and make your code modular, readable, and reusable.

How to Define a Function in JavaScript

You can define a function using the function keyword followed by a name, parentheses (), and a block of code inside curly braces {}.

function greet() {
  console.log("Hello, welcome to JavaScript!");
}

How to Call a Function

After defining the function, you must call or invoke it using its name followed by parentheses.

greet();

Hello, welcome to JavaScript!

Function Parameters and Arguments

You can pass data into functions through parameters, and use arguments when calling them.

function greetUser(name) {
  console.log("Hello, " + name + "!");
}

greetUser("Asha");

Hello, Asha!

Return Statement in Functions

The return keyword sends a value back to the place where the function was called.

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

let result = add(5, 3);
console.log(result);

8

Function Expression

Functions can be assigned to variables. These are called function expressions.

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

console.log(multiply(4, 2));

8

Arrow Functions (ES6)

Arrow functions provide a shorter syntax and lexically bind the this value.

const divide = (a, b) => {
  return a / b;
};

console.log(divide(10, 2));

5

When there's a single return statement, you can make it more concise:

const square = x => x * x;
console.log(square(6));

36

Default Parameters

You can assign default values to parameters.

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

greet();       // Uses default
greet("Ravi"); // Uses given argument

Hello, Guest
Hello, Ravi

Rest Parameters

When you don't know how many arguments you'll receive, use the rest operator ....

function sum(...numbers) {
  return numbers.reduce((acc, curr) => acc + curr, 0);
}

console.log(sum(1, 2, 3, 4));

10

Callback Functions

A callback is a function passed as an argument to another function and executed later.

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

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

process(done);

Processing...
Done!

Immediately Invoked Function Expression (IIFE)

IIFEs are functions that execute immediately after definition.

(function() {
  console.log("IIFE executed!");
})();

IIFE executed!

Understanding Scope in Functions

Functions define their own scope. Variables declared inside a function are local and not accessible outside.

function showScope() {
  let message = "Inside function";
  console.log(message);
}

showScope();
console.log(message); // Error

Inside function
Uncaught ReferenceError: message is not defined

Closures in JavaScript

A closure gives you access to an outer function’s variables even after the outer function has finished executing.

function outer() {
  let count = 0;
  return function inner() {
    count++;
    console.log("Count: " + count);
  };
}

const counter = outer();
counter();
counter();
counter();

Count: 1
Count: 2
Count: 3

Pure vs Impure Functions

Pure functions always return the same output for the same input and have no side effects.

// Pure
function pureAdd(a, b) {
  return a + b;
}

Impure functions may change external state or depend on it.

// Impure
let counter = 0;
function impureAdd() {
  counter++;
  return counter;
}

Summary

• Functions are core to writing reusable code in JavaScript.
• There are multiple ways to declare functions: declarations, expressions, and arrow functions.
• They support parameters, return values, rest arguments, and callbacks.
• Closures and scopes are essential for managing variable access and function behavior.

Practice Challenge

Write a function called greetTime(name, time) that returns:

• "Good morning, Alice" if time = "morning"
• "Good evening, Alice" if time = "evening"
• etc.

This will help you practice if-else and string concatenation inside a function!