Cleaning Up Your JavaScript With Functions

Functions are one of the fundamental building blocks in JavaScript. A function is a JavaScript procedure—a set of statements that performs a task or calculates a value. To use a function, you must define it somewhere in the scope from which you wish to call it.

-MDN

Functions are essential for keeping code DRY and making things easily reusable throughout the project.

Basic function syntax is something along the following lines:

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

function myFunction2() {
	var a = 0;
	return a;
}

functions can also call other functions:

function myFunction() {
	var a = 0;
	return a;
}

var num = 2;

function myFunction2(a, b) {
	return a + b;
}

myFunction2(myFunction(), num); // 2

Anonymous functions exist and are called at runtime without any named identifier. They are flexible and can be assigned directly to a variable. There are two forms, and they look like this:

// Function Expression
var squared = function (num) {
	return num * num;
};

// Arrow Expression
var cubed = (num) => {
	return num * num * num;
};