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Welcome back in today's lecture, are we going to talk about JavaScript variables we are going to see

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about variables, alert function, prompt function and undefined data types or JavaScript types?

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Remember, that was from the first lecture.

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And I've told you that we need to do this later in the course.

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OK, so open your vehicle, start new lecture, call it lecture, dash three JS.

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Make sure that you link it here first.

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Copy this one.

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OK, then we need to comment it out and make sure that you changed it to three.

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OK, now our browser should be like this.

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Actually, we don't need that one.

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No.

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Variables are something that is very crucial for JavaScript.

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So let us to store values inside this variable.

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So consider them as a container so we can add some data, we can store something inside.

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So this way, we are giving the program a memory where we can store data.

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So variables hold.

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Any JavaScript data type so they can hold any data that we have just covered.

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OK, so let's call it here variables.

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JavaScript variables helps with having variables, and so first things first.

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We need to first specify that we are talking about the variables and how are we going to do this, we

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we are.

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And then there is goes the name of the variable and there we can specify the data.

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And now this.

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Another operator comes to place equals means we are assigning data here.

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All right.

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So how we can print out the variables to consider that log name.

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Let's call here my name and take notes, something that I did here and I will just explain a little

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bit later.

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So go back to your browser and here it is.

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Let's change it to more meaningful, Rick, or you can put Andy doesn't matter your name, so why variables

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are so useful.

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OK, let's do it like this.

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And I'm going to show you, First Consul, that Locke.

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Five plus nine and save it, so that will give us a result 14, but if I want to access the same result,

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I need to copy the same line of code and execute it here.

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Right.

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So that will give me to fourteens.

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Well, this is not ideal.

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This is called code repetition.

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And there is a rule here in not just in JavaScript.

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Or any other programming languages, it's called dry.

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Do not repeat yourself, so it is a good idea to find a solution how we will not be able to repeat ourselves.

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So that's where variables comes into place.

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Now, what we can do here, I'm just going to recommend this one, for example, variable.

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Let's put it here somewhere and I will put five plus nine.

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OK, now if I go back here, there is nothing there, right?

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But I could call it log just some because the very variable, some holes in the memory, this data 14.

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Right.

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And if I create another variable called result, then I can still use this some some minus five for

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example.

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And that should give me.

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Nine.

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And I need to also consulate like the result.

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So now you see the power of variables, we can we use them because they're dear, they're defined,

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they're they have a value.

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And now we can use them in the course of if I want, I can just again use the result and I say.

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Let's make it here.

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This is the result from the variable.

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Semicolon there, plus make a breathing space, make sure that you close the semicolon and there you

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go.

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Now I have nine again.

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I have nine here so I can reuse it.

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That's called reusability.

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So that is pretty much it.

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And now let's talk about the part here, the notation, the way we are just giving the variable names.

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Now, it is very important for us as a developers to create good variable names.

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A variable should accurately identify a variable name, actually should accurately and identify your

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variable.

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Right.

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For example, if I say my name, then you should as a developer from outside to expect that, well,

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this will hold a string with my name.

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Right.

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But what about this notation?

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It's going to call it's going to be called cavalcades notation in how this notation works.

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Now, when we have a variable that is built from two words like first.

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Name, then we are going to use the Campbell case notation and how this Camil case notation works.

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Now, the Kemakeza notation works.

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The first word is going to stay with a small first letter.

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So it's not going to be capital letter.

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But the second word will have a capital first letter and it will be this first name.

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So throughout the course, I will use Camil case notation and you should also do the same thing, but

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in future.

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If you see somewhere or in some drills that they use some other notation, then that is OK, it's going

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to work.

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Now, another rule, how we can name our variables is that the very variable names cannot contain spaces.

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For example, variable, my name, it's not going to work.

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It's contained spaces.

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Another thing that the variable you should know that they must start with a letter on their score so

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they can start with the underscore and you can continue with combinations of letters and numbers.

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Right.

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So you can even have a numbers.

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You can start with underscore.

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We can start with the letter or you can start with a dollar sign.

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So let's do it like this, where names can start with letter underscore or.

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Dollar sign, right, that is pretty much it.

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Now, another thing is you should know that variables are case sensitive.

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What that means, variable A B equals.

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Eight.

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And variable A B equals eight are very different, why they're case sensitive, this is a capital B.

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This is a lower B, so they're D variable is completely different than this.

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Now, in the memory, we have two different variables, but they also have same value.

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But that's different.

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And yeah, so variable can contain letters, numbers underscore or dollar signs.

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They're case sensitive and they can start with this three.

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So you cannot start variable with one or you cannot start variable with a special symbol like this.

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And that or even the special symbol will not be able to be applied here.

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So if you save it like this, save it, you will have wait.

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You'll have unexpected token.

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And that was the token that we just put.

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Nice.

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So let's continue working with our examples.

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So what I want to show you that for the first time we will be able to use a function.

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So these functions are inbuilt.

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So we have access here.

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For example, the first is prompt and if I do it like this, F stands for function.

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So it tells me that is a function.

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But every time I use function, I need to explicitly write these brackets.

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So cancel.

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Actually I don't need to cancel the log now in semicolon again.

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Now if I go back to a browser then the browser will give me a pop up where I can put a data or string

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here.

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And if he did, OK, now there will be stored somewhere.

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Prompt.

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So, again, I will not be able to access the data from the program.

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Why is that?

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Well, because I have in store it in variable, but what I can do is variable data.

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That then I haven't told you this, you can write here, what is your name and make sure that this I's

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capital looks better.

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Now look what will happen here is what is your name?

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But the browser is still there.

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So, Rick, now, if I hit OK or return, then again, I don't have access.

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No, I'm just joking.

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So the data variable is here and yeah, I just have access to whatever.

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We just enter through that prompt.

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So why I just access that data here and then right here.

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Console the log in that data.

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Well if I do this in I will save it.

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Then the entire life server will kick in every other line.

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It will go line by line, the compiler and it will delete whatever had previously.

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Hold it here, store it here in this data.

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That's why sometimes it's very useful, this browser console, so I can just use it.

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That's pretty much it.

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Now let's do something interesting.

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Another function that I want to show you is called alert, but let's spice this one up.

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So I'm just going to prompt the user and I will say, how old are you?

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Are you and I'm just going to copy this page that now I'm just going to ask about the year and what

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I'm going to say.

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What is the current?

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You're nice and I'm going to use a learned function for the first time.

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Now, this alert, I don't have to save it in a very old I can, but I don't have to.

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And I will say I was born.

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And and semicolon here, space it plus now I don't want to confuse this function, what I will do,

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I will get this in a brackets and I will just minus a year with the H.

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So why just go with the year will here?

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It will be bigger than the H obviously.

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And that will never produce a negative result.

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So this is a pure mess.

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So save it.

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Go back to your browser.

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How old are you.

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Thirty three.

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I don't know if you can see this.

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How.

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Let me just zoom in if I can.

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I can actually.

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So thirty three here.

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OK then.

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Twenty, twenty two here.

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OK then.

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I was born in this year.

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Nice.

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That is pretty much what the alert function can do in there, out of the box there for us so we can

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use it.

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So this is to alert the user that something is happening.

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For example, you have you have failed to log in or your password and confirm password does not match.

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There are some scenarios that we can use all of this and they're pretty much cool.

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When I started JavaScript.

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I was I was like using this problem in my in my lectures and in my exercises.

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And I was pretty much, you know, excited because, oops, I am I'm doing something with JavaScript,

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so that's pretty much it.

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And the last thing that I just want to talk is the undefined.

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JavaScript is type.

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Now, this is from the first lecture.

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Right, we just get Boolean true, but we didn't finish with the rest of them.

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Why?

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Well, because, for example, if I do hear variable.

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Last name in semicolon and save it and try to conceal a log, last name, so I'm accessing the last

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name, but I didn't initialize the value, so maybe I haven't told you what variable my name is or variable.

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Any name here is saying, OK, we are going to declare variable with this name and we're going to assign

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value after that.

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So let me see.

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Let me show you what will happen.

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Oh, now it's a good idea to go back here.

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Just to comment out this part, because it will ask you the problems all over again.

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You don't want to see it, so it's going to say undefined.

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So why we are having undefined and this is one of the data JavaScript types that we need to discuss

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now.

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Undefined is meaning that we don't have any value in that variable.

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So undefined is that when we try to access something that doesn't hold data.

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So, yes, there is a memory triste of our variable called a last name, but there is no data.

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So it's only memory reference doesn't hold any data.

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And that's why when we try to access it here, it gives us this undefined.

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Now, when I haven't shown you is that you can also later on before you use this last name, you can

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say you can actually initialize you can give a data and now it will work.

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It will say Sokolsky.

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Why?

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Because some counseling log after I declare and initialize with the data this variable that is simple

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as it can be.

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Now, that is pretty much what I want to show you.

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And as a lasting, um, it's good idea to mention the semicolon.

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So one thing that we haven't covered is the semicolon that I put at the end.

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So semicolon marks the end of the expression.

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Anything that is written as a value like this is considered as expression.

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Now, it will be working without Collo nowadays, but semicolon.

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So it will be working even without semicolon.

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But it should also.

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Very good point for you to just start using the semicolon because that will end the expression.

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And yeah, it's a very useful MI, as is a teacher, to teach you that you should use it and that will

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stay with you.

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And you can later on in your programming you will use semicolons.

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So that is pretty much it.

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Now semicolon is anything that is written as a value, is considered as expression and semicolons puts

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always at the end of that expression.

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And that is pretty much for this lecture.

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We haven't done much and you can come at this one now, but it is what it is.

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Make sure that you do the exercises, so I will see you in the next one.