Now, functions in JavaScript are one of the most critical topics related to learning how JavaScript works and how you can build full-fledged programs. If you've never heard of a function and you're new to programming, the way a function works is its kind of like a machine.

This machine has the ability, you can think of a machine on an assembly line or some kind of plant, this machine has the ability to take information in and then it performs all kinds of processes. These are processes that you tell it to perform and then it returns a different value.

So it takes in some kind of value and then it returns a different value. If you can understand that core concept related to functions, it's going to help you as you learn JavaScript as a whole. I once had a teacher tell me that if you can understand an input and an output for a program, you can understand anything. That is definitely the case when it comes to functions.

I think one of the best ways, before we get into the code, of understanding how functions work is to look at a real-life example. Let's imagine that you're building out a website that has authentication, which means that it's like pretty much every Web site that you go to like Facebook or Twitter where you have to type in your email address and then your password.

Then if you typed in the right combination: they'll let you in, let you see your profile, and go through the application. Well if you were to build that out. Imagine a scenario where you need to build that out from scratch.

There are a lot of processes that have to take place: you have to connect to a database, you have to be able to read data in, so you have to read data in from the form, then you have to perform a number of conditionals to make sure that the email address is correct.

That it's in the database, that it matches the password, and then from there you have to redirect the user. If they entered the right information to their profile page, and if they entered the wrong information you have to let them know that they typed in the wrong username or password.

Well, that's a lot of steps, and so what a function can do is it can wrap all those up into a single line of code. What you can do is imagine if you had all those steps, say it's a dozen steps you needed to authenticate a user, if you needed to log users in on multiple pages. If you didn't have functions you'd have to copy and paste that code into every single file that you wanted to authenticate the user in.

That is really a bad practice because then imagine a scenario where you have to change one of those lines of code. You'd have to remember every single place that you entered that code in and then you'd have to go and make that change. That's very error-prone, and you really would not have a good time programming following that process.

What a function allows you to do is you can take all of that code, each one of those steps that were involved for logging in the user, place them inside of a single function, and then anywhere else in the program that you want to see if a user can log in or not: you simply call that one function.

You pass in the email address, the password, and then the function then is run automatically every time that you call it. I know that may seem a little abstract if you've never worked with it before, but I think it helps to have a real-world scenario on when you'd want to use a function.

Now that you have a high-level idea on how it works. Let's get into the code and let's start building out our own functions in JavaScript.