JavaScript Functions

Functions are one of the fundamental building blocks in JavaScript. A function is a reusable block of code designed to perform a particular task. Functions can take inputs (parameters) and return an output (result).

Defining a Function

A function can be defined using the function keyword followed by a name, a list of parameters (optional), and a block of code to execute.

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

// Calling the function
greet(); // Output: Hello, World!

Function Parameters

Functions can accept inputs called parameters. These parameters allow you to pass data to the function when it is called.

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

// Calling the function with a parameter
greet("Alice"); // Output: Hello, Alice!

Return Values

A function can return a value back to the caller using the return statement.

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

// Calling the function and storing the return value
let result = add(5, 3);
console.log(result); // Output: 8

Function Expressions

Functions can also be defined as expressions and assigned to variables. These are called function expressions.

const square = function(x) {
    return x * x;
};

// Calling the function expression
console.log(square(4)); // Output: 16

Arrow Functions

ES6 introduced a new way to define functions using arrow functions. Arrow functions are a shorter syntax and do not have their own this value.

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

// Calling the arrow function
console.log(multiply(2, 3)); // Output: 6

Immediately Invoked Function Expressions (IIFE)

An IIFE is a function that runs as soon as it is defined. It is often used to create a new scope and avoid polluting the global namespace.

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

Functions as First-Class Citizens

In JavaScript, functions are first-class citizens. This means they can be stored in variables, passed as arguments to other functions, and returned from functions.

function executeFunction(func) {
    func();
}

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

// Passing a function as an argument
executeFunction(sayHello); // Output: Hello!

Understanding how to define and use functions is crucial for mastering JavaScript. Functions help you organize your code, make it more readable, and enable you to reuse code efficiently.