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Welcome back.

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So in this lesson we get to cover one of the most important concepts in all of programming.

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And in fact I might argue that it's actually the most important concept which is conditionals.

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And the reason I say that conditionals are so important is that they are the one way that you add decisions

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to your code.

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It's how you add logic to your programming so give you a few examples of real world uses for conditionals

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.

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Let's take an example of user logon.

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So when you go to log in on a website you type a password in that form and there is code that checks

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that password that you typed in against the password in the database.

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And if they match you log into the site and you get redirected to another page.

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And if they don't match you usually get an error message or some sort.

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So there's two different paths given the same exact code.

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Another example would be when you buy something online when you type in your credit card information

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.

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If it works if the transaction goes through you might get a confirmation email you see a success screen

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.

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And if there's some problem with your information or you don't have funds in the account you're going

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to get an error message.

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Again two different outcomes.

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So now let's go see conditionals in action.

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So there are three key words that I need you to remember throughout this lecture which are if else if

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and else.

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So these are the three javascript conditional keywords and yes I know LCF is technically two English

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words but to javascript that is just one key word.

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So the example that we're going to use to illustrate conditionals is that of a bouncer at a concert

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venue.

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So the idea is that we're going to have an age variable and if that age if a person is younger than

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18 Unfortunately that person cannot get into our venue if that person is older than 18 but they're not

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yet 21.

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That means that they can enter but we have to draw an X in their arm or in their hand because they cannot

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drink.

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And then lastly anyone else if you're greater than 21 you can get in and drink.

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So those are the three things that we're going to try and write with code.

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So before we move on we're going to do a quick recap of the Boolean logic operators from the last video

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.

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And the reason that we're doing that is that we actually use our three keywords that I mentioned at

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the beginning if LCF and else in conjunction with these boolean operators to make our decisions to add

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the logic.

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So to recap we've got less than greater than less than or equal to greater than or equal to.

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Pop quiz here.

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What's the difference between these two.

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So Pozzi video and try and remind yourself the difference between chirpily calls and double equals.

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So the difference is that double equals performs type coercion while Tripoli equals as strict equals

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.

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So type coercion.

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Let's start there.

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Double equals means that the number one is actually going to be equal to the string one.

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When we used double equals because javascript does not care about type.

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However when we do triple equals which is strict It does care about type and value together.

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So one is triple equal to the number one vote one would not be triple equal to the string one.

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And then we've got these two operators and and or.

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And remember that and is only true if both sides are true however or is true.

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If one of the sides evaluates to TRUE.

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OK so now let's actually move on to writing our logic.

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So the first thing we're going to see is the if key word the if keyword takes a single expression instead

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of parentheses which it will evaluate to be true or false.

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And if it evaluates to be true whatever code we provide inside the curly braces in this case a console

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like statement will be executed.

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So this constant alt log will only be executed well it will only be printed out if age is less than

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18.

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If it is greater than 18 if it's equal to 18 we will not see this text printed up.

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So that's the first part that's our bouncer checking if you are too young to get it.

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So next up we're going to learn about else.

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And the way that LCF works is that it follows an if statement.

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It is a secondary condition.

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So in this case we're checking if age is less than 18.

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So if someone is too young to enter the venue if they're less than 18 they can't get in.

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The next thing to check only after we check the first one is are they older than 18.

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But less than 21.

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So LCF works the exact same way as if we pass an expression in parentheses that will evaluate to be

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true or false if it's true.

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The code inside the curly braces will be executed if it is not true if it's false nothing happens.

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So again LCF works just like if.

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Except that it must follow in their statement.

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So we have this line here.

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Age greater than 18 and age is less than 21 which is actually a little bit redundant.

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So think about this for a second.

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Plus a video and try to identify how we could refactor this.

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So this is a really minor change but you'll notice I got rid of if age is greater than 18 right here

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.

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And less than 21 and just turned it into if else LCF age is less than 21.

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So again just to reiterate we had this before and we refactored it into this.

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And the reason that we did it is it's redundant.

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The only way that this code is going to run is if this first if statement is false.

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So what that means is that this code here will only run if already we've established that age is not

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less than 18 or that is greater than or equal to 18.

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So if you are greater than or equal to 18 and less than 21 you can enter but you cannot drink.

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So the last step is the.

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Otherwise section which is.

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Else so else allows us to just have something else happen any other time.

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So in this case if you're not less than 18 and you're not between 18 and 21 that means that you can

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come on in and you can drink and so else does not take an expression in parentheses.

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It's different in that we just write else and then whatever we put in said it curly braces will only

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be run as a last ditch effort.

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So this will be run first and if nothing is printed out if this is not true then we'll move on to this

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.

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If this is not true then we'll move on to this.

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But remember if along the way either of these is true then everything stops and this else will not run

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.

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So if this is true this code is done.

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If this elf's if is false.

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But this is true or else never runs.

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So remember these are three different paths for one decision here.

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OK so your exercise here there's a few of them is to take the code that I just gave you with the bouncer

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for a concert venue.

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And to add a few let's say quirky other conditions.

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So if age is net negative we want to make sure that we print an error message.

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If age is 21.

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Exactly.

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We want to print happy 21st birthday give them a free shot.

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And if age is odd you should print your age is odd.

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So you have to do a little bit of math there to figure out if it's odd.

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And then the last one which is a bonus is to figure out if age is a perfect square and if you don't

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remember what a perfect square is you can look that up but it's basically a number whose square root

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the two factors are the exact same number.

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So four times for 16 so 16 is a perfect square four is a perfect square because two times two is four

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.

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All right so that's a super bonus.

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Have fun with this and I'll see in the next video.