Build a basic function

- [Voiceover] Let's build a simple, named function and see how all of this works over the next couple of movies. In the exercise files for this movie, 0402, you'll find our trusty old blank index.html document and an empty script.js file. I'm previewing both in my browser, using atom-live-server and have the console open. If you don't have atom-live-server, or you don't want to use it, you can get the exact, same result by just reloading index.html in your browser. Now we need a reason to create a function. Fractions can be really hard to compare, so I want to create a function that quickly tells me what fraction is bigger, so I can ask it things like, "Which is bigger; three over four or five over seven?" We start off by creating the function itself - "function", then give it a name, "findBiggestFraction()", and then, just to make sure everything works, I'll console.log out "The function is running!" Save that and nothing happens.

That's because, although we've defined the function, we have not actually made it run. To get the function to run, we have to call it by its name: "find" -- and you notice here my quote editor is trying to help me, so it says, "Hey, you just created a new function called findBiggestFraction, is that what you want?" If it is, just hit Enter. Quote editor completes the quote for you, this is amazing for people like me, who keep misspelling things all the time, but it also just reminds you, maybe you already have a function by this name, if you're defining a new one. Now that I've called a function, when I save, everything runs properly and we get the console log, "The function is running!" Awesome! So far, so good.

Now we need to get the function to compare some fractions. To start off, we'll define two new variables that actually hold the fractions: I'll call them a and set up three over four; and b which is five over seven. These are the two fractions I want to compare. Then, I set up some logic inside the function to compare the two, and here I'll use a ternary operator, so I'll say, "If a is bigger than b," then console.log the text a: and then the value of a, else console.log text b: and the value of b." Save that, and we get the result right away.

The biggest fraction is a, at the value of 0.75. Now, pay close attention to the order of the contents here. I define the function first, and here I'm using the variables a and b even though they haven't been defined yet. Then I define the variables a and b and then I call the function. The reason why this works is until I call the function, it's just sitting in the memory of the browser. That means as long as I define my variables before I call the function, everything will work fine. However, if I take these variables and move them down below the function call and then try to run this, I get "b: undefined".

That means it's trying to find the function and it goes, "I don't have a variable called a so "I'm just going to make one." Remember how I said if you don't define your variables and then just call them, JavaScript automatically makes them? That is exactly what is happening here. So, because I define my variables too late, everything breaks. The order in which you place things in JavaScript really matters. Alright, we have a function that works. In the next movie, we'll expand on it.