Modern Functions in JavaScript

Functions are reusable blocks of code designed to perform a particular task. They are executed when something "invokes" (calls) them.

In modern JavaScript, there are two main ways to declare functions: Traditional Functions and Arrow Functions.

Traditional Functions

They are declared using the function keyword, followed by a name, parentheses () that can contain parameters, and curly braces {} enclosing the code to be executed.

// Declaration
function greet(firstName, lastName) {
    return `Hello, ${firstName} ${lastName}!`;
}

// Invocation (call)
const message = greet("John", "Doe");
console.log(message); // "Hello, John Doe!"

Anonymous Functions

In JS, functions are "first-class citizens", which means they can be treated like any other variable. You can assign an unnamed (anonymous) function to a const variable.

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

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

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Arrow Functions

Introduced in ES6, "Arrow Functions" provide a much shorter and concise syntax for writing functions, and they also change the way the this context behaves (a vital advanced topic in frameworks like React).

Basic syntax: (parameter1, parameter2) => { code }

// Arrow Function equivalent to 'add' above
const addArrow = (a, b) => {
    return a + b;
};

The superpower of conciseness (Implicit return)

If your arrow function only consists of a single line of code that returns a value, you can omit the curly braces {} and the word return. The return becomes implicit.

Furthermore, if the function receives exactly one parameter, you can omit the parentheses ().

// 1. Original syntax
const multiplyByTwo = (number) => {
    return number * 2;
};

// 2. Omitting parentheses (only 1 parameter)
const multiplyByTwo = number => {
    return number * 2;
};

// 3. IMPLICIT return (Only 1 line of code)
const multiplyByTwo = number => number * 2;

console.log(multiplyByTwo(5)); // 10

This ultra-short syntax (number 3) is what you will see in 90% of modern JavaScript code when using array methods (like .map or .filter).