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Welcome back! Functions are the hardest part of JavaScript.

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And it will take some time to fully get it. However,

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in this video we're going to try and do that.

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We're going to learn JavaScript functions.

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So, let's start off with, "What are functions?"

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Well, they're piece of code, that perform actions. Without functions

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JavaScript wouldn't really do anything.

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The beauty is that functions can perform one action or multiple actions. But we'll get back to that. First,

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let's talk about how you already know some JavaScript functions without even realizing it.

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You've seen them before.

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'alert' - that's a function. 'prompt' -

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that's a function.

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You've seen these before.

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So, JavaScript provides for us some functions, that are really-really useful, so that everybody who writes

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JavaScript, can use them without having to write them over and over.

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So with functions we have 2 options.

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One - is using existing JavaScript functions, like 'alert' and 'prompt'.

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And two - we can create our own.

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So, before we get into how we create our own functions and what is so useful about them, you might have

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noticed - noticed one thing: these brackets, that are right next to each other with functions.

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What do they mean? Well, that in JavaScript means to execute the function.

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So, let's see if what happens, when I just type in 'alert' and press 'enter'.

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I get 'f alert()' and some weird stuff. Hmmm, 'f' over here stands for function and can see when I hover

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over it.

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It says 'function alert()'.

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This is what running a function means: in order to run

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'alert()'

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I execute it by calling it like this ['alert()']

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So, these brackets mean 'calling a function', so I can call a function.

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And you also noticed, that within 'alert' I can add things like

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"hi there!"

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These are all called 'arguments' and I know, I'm throwing a bunch of jargon, a bunch of words at you, but these

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are just things that you'll hear over and over,

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when you're a developer, and they'll stick to you - it won't the first time, but, like I said, just try and

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get there one step at a time.

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So 'arguments' are what's given to functions.

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For example, when we did, in our example here.

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"Hellooooo" and the 'console.log' was an 'argument'. And the interesting thing is that you can have multiple arguments:

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for example, if I removed this and just have, let's just have 'console.log' for now.

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I can do "How are you".

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I save and refresh and I get.

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"Hellooooo How are you".

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See, I can have multiple arguments, but just adding a comma. But again, that depends on the function.

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Now, again before we start creating our own, let's review. 'alert' is a function, 'alert' with the brackets,

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I'm calling the 'alert' function and I am calling the 'alert' function with the argument

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'Hi'. 'alert' needs to be called,

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otherwise it's just sitting there.

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Now the big reason to use function is that you can call them as many times as you want.

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Think about 'alert', imagine having to say every time "Hey, computer, create this 'popup' window and print whatever

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argument I put in" every time.

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I mean that would really suck. Isn't it nice, that we can just call 'alert' and that popup window pops up for

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us and everything's done for you?

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Well, that's pretty much what functions do.

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They make life easier by having actions, bundled up, so you only write them once and then you can just

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call it.

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OK. So let's get to the fun part.

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Let's create our own functions. So, if we go back to our document here, let's see right here. We have 2

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ways of creating functions and I'll show you the first way.

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Actually, I'm going to show you this one first.

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So, let's create our own function, using the first method, which is 'function declaration'.

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And we're going to use our 'script.js' here, that's linked to our HTML, that we did at the previous video.

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So, here I can say 'function sayHello()'

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is 'console.log("Hello")'.

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Again the syntax a little bit different, but you just have to remember it.

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I'm saying 'function' - is going to be called 'sayHello'.

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We have those brackets, that are very familiar to us. And then 'console.log("Hello");'. Let's see, what happens

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when I save. I refresh.

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Hmmm, nothing happens.

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Why is that?

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Well, we declared this function.

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But remember, what we did with 'alert'? Well, it exists, but we haven't run it.

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We have to call the function, so we have to say 'sayHello', like so. Now when I refresh.

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I get "Hello".

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So think of this with the variable. When I say 'var a equals 5',

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that's not going to do anything, until we actually get the 'a'.

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And that's when it prints '5'.

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OK. So that's one way.

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What's the second way of creating a function?

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The second way is called 'function expression'.

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And this goes like this.

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I can say 'var sayBye = function()' 'console.log("Bye");'.

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Again I'll save this and refresh.

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And again we only get 'Hello' because, well, again we have to call that function.

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So, I say 'sayBye', run it and refresh. "Hello", "Bye". Now

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one thing you may have noticed here, is that we're assigning this function to the 'sayBye' variable, but

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what is the name of the function?

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I mean here clearly function's name is 'sayHello', but here we're, we're just assigning to the variable.

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I mean, technically the function doesn't have a name and this is called an 'anonymous function'.

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We've assigned this function to 'sayBye', so we can reference it through 'sayBye', but otherwise we can't

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really access this function.

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Well, you'll see why that's important

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in later videos.

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But you could technically do this: and call this function 'byebye'.

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OK.

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So, now that we know, how to create functions, why do we need these arguments?

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Remember, why do we need stuff inside the brackets?

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I mean - these look useful enough, right?

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Well, let's try something. 'sing', let's do a function 'sing' and this 'sing' will have, will have 'console.log'

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and I'll say "AHHHHHHHHHHHH". That's, that's me, singing.

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And it also has 'console.log("TEEEEEEEEE")'.

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That's how I sing. Now, if I save this and refresh, make sure it still sings. Oh, I have to call it!

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Let's refresh.

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Beautiful song: AHHHHHHHHHHHH, TEEEEEEEEE.

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But now, every time I want to change to a different song, maybe I wanted to say 'laaa deeee daaa'.

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Well, I have to either delete this and change this to 'laaa deeee daaa' or I have to create a new one, calling

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'sing2' 

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'console.log' and you can see over here, how annoying that is, that I'm typing the same thing over and over

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and just changing these things.

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And now I have to run 'sing2'. And I refresh, and phew, that was tiring! OK.

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I did all of this and now imagine, if we had thousands of songs, that we want to sing; we're 'Spotify' and

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we want to display all the song lyrics.

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I mean that's pretty-pretty tiring, right?

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And one of the rules with developers is this idea of "DRY: Do not repeat yourself".

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Ideally you want to make things as efficient as possible.

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And one thing you can do is - by adding arguments. So I can add something like 'song' and now - let me just

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delete this for now.

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In the 'console.log' I can just say 'song'.

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So now any time I want to change the song, I can say "Laaa deee daaa". 'sing("helllloooooo")

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Then "sing

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("backstreets back alright")

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Now I save and refresh.

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And look at that! You see how many lines of code we just saved and how I can now use 'sing' any time I want,

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and I can customize it to what I want.

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Kind of like 'alert'.

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That's what arguments do.

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Arguments allow us to not repeat ourselves and make our functions what we call more extensible.

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They can be customized.

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Let's do one other thing to learn a fun thing about functions.

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We're going to delete this and try 'function multiply()'. And we'll give it 'a' and 'b', so it'll accept two

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arguments and we'll do 'a*b'.

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And now within 'a*b' we'll say 'multiply', and we'll call 'multiply(5, 10)'. Save and refresh.

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And now we should have 'multiply' available to us.

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You can see over here 'multiply(a, b)' that's 'b' exactly what we wrote.

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OK, so what happens if I do 'multiply'

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(5, 10)?

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What do you think I'll get?

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Let's see!

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Hmmm, that's really weird.

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Let's try that again with a different number.

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Still 'undefined'.

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Why is that? Well, this is called debugging.

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Let's see if we can figure out what's happening here.

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I'm going to do a 'console.log' here.

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And I'm going to log out 'a' and 'b' to make sure that this function's actually running.

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So I'm going to save and refresh. OK. 'console.log', 'a' and 'b' and I just run the function again.

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'console.log'. OK. So it looks like it's running, but then I'm getting 'undefined' over here.

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Well, I can show you this in a diagram.

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So, a function is an 'input'.

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We give it some sort of an 'input', maybe sometimes it's empty, so that's whatever's in the bracket.

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In our case it's 'a' and 'b'.

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So those are numbers, so '5' and '10'. And then the function does whatever we tell it to do.

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In our case it was to 'multiply 5 and 10'. And then we get an 'output', but we're not getting that 'output'.

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Why is that?

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Well, because a function works like this.

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And don't worry, I've just added a bunch of arrows, but I'll go through everything and explain it to you.

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The 'input' is receiving '5' and '10'.

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The function does some stuff, and up to this point we've only done 'console.log', and the way 'console.log'

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works is, the function is saying: 'Just log it to the console in the browser'.

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But we've never done it where we've returned a 'value'.

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Let me show you what I mean.

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When we don't return something, we get 'undefined'.

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When we return something, we get the 'value'.

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So let's see over here if we can explain this.

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So, here whenever I am saying 'multiply (10 ,5)', and I'm going to remove the 'console.log' just so it doesn't confuse

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us, save and refresh...

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I get 'undefined'.

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That is because as you see in this diagram we're not returning anything.

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So, in JavaScript the way you return things is - you have to put in 'return' inside of a function.

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So, now it's going to 'return a * b' and you see, that it's a - it's a special word in JavaScript - it

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highlighted in red.

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Well, now it's saying 'Yep, we're going to return the value'.

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So, even if we added 'console.log'. You'll do 'console.log' and then ask "Hey, do I have a 'return'?".

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If I do, I'm going to send it a value.

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Hopefully, that's not too confusing.

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Let's see if it works.

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I'm going to save and refresh.

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And now if I do 'multiply (5, 10)', I get '50'.

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Yay! We got our function working and that is a very important key concept, that we need to remember

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is that we need to return something from a function.

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I mean we don't have to.

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But ideally we do, so we don't get these cases, where we just have 'undefined' and we don't know what the

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function does.

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It's kind of like a 'black box', that does something and we might get a 'console.log', or not but we don't

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know what's happening inside.

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It's nice to have a 'return' to make sure that the function acts the way we expect it to.

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Ok, what if we do something like this?

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What if I do 'return a', and I forgot a semicolon here, and 'return b'. What do you think will happen? Let's

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refresh and run this.

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OK.

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What if I change the order and I put 'a' first?

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Let's try that again.

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I get '5' and 'return' is the final way to end a function if that makes sense. So, as soon as you say

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'return' in a function, the program exits.

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So, to go through this I say 'multiply (5, 10)'. It goes to the function, it says: "Yep, I have the function

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'multiply'. I'm going to put 'a' as '5' and '10' as 'b'".

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And now it goes into the function and says: 'return 5', which is 'a'. So it returns that and exits the function.

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So now these two lines aren't being run at all.

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The program simply reads this, goes to here, reads the first line of the function and then exits.

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So, in a function you should have one 'return' statement, or is that right?

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Let's see a case where that might not be correct.

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Let's add an 'if' statement to this.

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Let's say that we want to do 'multiply', but we're also kind of lazy and we don't want to be too hard with

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the computer.

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So let's say we want to say that, 'if' - we remember the 'if statement'. "if 'a' is greater than '10'" or "if 'b' is

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greater than '10'".

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We can say 'return "that's too hard";'.

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Otherwise [else]

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we'll return 'a * b'. Let's see what happens here.

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I'm going to save and refresh. Let's do 'multiply (5,4)'.

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I get '20'.

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What if we do '5' and '40'? I get

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"that's too hard", because now it's reading the function '5' and '10', and it's saying: "Hmmm, 'b', which is '40' is higher

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than '10'".

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So, according to the 'if statement' I'm going to go "that's too hard".

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And because a 'return' exits the function, it never gets to this, even if I do 'return a * b', you'll never

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get there, because the 'return' exits the function. Just to double check,

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let's do that: let's do '5*40'.

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And it never gets to any of these lines, because as soon as this sees a 'return', it exits the function.

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I know it's a lot.

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And we'll get to why these things are important in the next couple of videos.

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But I wanted to also show you that you can have inner functions. So in JavaScript functions are 'variables'.

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And what that means is, that we were able to assign

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functions like this. Right?

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And if we did something like this,

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well, we can call 'a' in the same way that we did and we just assigned function as a variable.

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So technically we could do something like this.

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So let's have 'multiply' - Let's go back to the way we had it. We'll leave 'multiply' the way it is in the

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simple form.

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And we can actually

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say 'alert(multiply)'.

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And then here we'll do '3' and '4'. And I know that's a lot of brackets.

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Don't worry we'll go through it.

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But, let's just save this and refresh and see what happens.

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I get '12'. Because what's happening is: We're starting with the inner function and we're saying: "Hey, we

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want to alert something!".

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"What do you want to alert?"

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"Well, we want to 'multiply'"

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"We want to call this function and give 'a' it '3' and '4'".

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So it goes to 'multiply' and it says: "Yep, we'll assign 'a' and 'b' to '3' and '4', and I want to return 'a' and 'b'.

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So now 'multiply' gets changed to '12' and then we 'alert'.

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So, you see that now, instead of having something like 'total', 'multiply(4,5)' and then putting 'total'

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here, we can just assign 'multiply' into here.

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Whew! that was, that was a lot!

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But I want to show you that we've - we've tackled,

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I think, the hardest topic in JavaScript, which is functions. And you've also covered the 'return', which

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is very very good.

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The one other thing I want to show you is that a lot of people get confused with the terminology and

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I don't think it's as important but I just want to clarify it.

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There is the concept of 'parameters' and 'arguments'.

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Now 'parameters' and 'arguments' kind of are very similar but just slightly different.

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So 'arguments' as we've said, are '4' and '5'.

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So functions can have 'arguments' and they get called with 'arguments'.

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'Parameters' are what 'a' and 'b' is.

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So 'multiply' has two 'parameters' of 'a' and 'b'.

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I know that's a little bit confusing.

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You can read up on it a little bit more.

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I don't think it's important to really know the difference, but just so whenever you're reading articles

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or learning, maybe through YouTube, that you'll hear these words almost interchangeably.

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They pretty much mean the same thing, just a little bit slight difference in them.

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All right!

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I know, I know I've thrown a lot of terminology at you!

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But after repeating it a few times it will make sense, so stay strong. But that's it for functions.

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It's time for you to try some exercises.

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Rewatch this video if you need to.

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You really want to make sure you understand functions, since it will be the core of JavaScript.

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Just remember, what we're doing with functions is, we are creating new words in the language, in the JavaScript

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language. So we can create 'variables' or we can create 'functions' to add vocabulary to the language and

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up to this point that's all we've been doing.

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We got this JavaScript that had a few words, that we can use, and we've added new ones like 'multiply' and

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'total' in order to make it more useful to us.

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And that's what programming is.

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I'll see you in the next one. Bye-bye