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With numbers, strings, arrays, and objects,

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we now got four super important types of values

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that you will use a lot in JavaScript programming.

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Now let's build up on that knowledge.

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And one thing we haven't done too much,

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or actually not at all up to this point,

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is work with those values.

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Sure, we use the alert command to output them,

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but very often you want to do

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calculations, transformations.

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You want to do operations like that

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with your data.

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And therefore, I now want to explore

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some important basics related to that with you now.

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For this I'll delete these two alert lines here,

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and now let's say we want to derive

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for how many years this user,

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which we are describing here, is an adult,

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assuming that you become an adult

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once you reach 18 years.

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Now for that, we could add a new variable

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that stores our result of that calculation,

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which we could name adultYears.

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Again, the name is up to you

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as long as you follow the rules and the convention,

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but the name of course also should convey some meaning

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and describe the value which you will store

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in this variable.

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And here I plan on storing the number of years

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for which this person here already is an adult.

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Now, the interesting thing here now

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is that now I don't want to hard code a new value.

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Of course, I could store 14 in there

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because I quickly calculated in my head

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that 32 minus 18 is 14,

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but that's not the idea here.

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This input here could be coming from a user

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through a form.

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So even though we did hard coded here

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as a developer for demo purposes,

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this value 32

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might actually not really be known by us,

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the developer.

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And therefore, we want to derive

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the adult years dynamically.

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We want to calculate them.

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Also of course, to avoid the fact

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that we make an error

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whilst doing that math in our head.

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So therefore, instead of just the writing 14 here,

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I instead want to use my age,

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which stores the 32,

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and now deduct 18 from that age.

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And you can do all those basic math operations,

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which you know, in JavaScript as well.

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You can deduct something by adding the minus symbol

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and then the value you want to deduct,

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18 in this case.

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And now adultYears will store

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the result of this operation.

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And of course, this could also be a longer operation

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with multiple steps.

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So you're not just allowed

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to deduct one value from another,

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but you can add as many steps here as you want.

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Of course, you're also not limited to subtraction.

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You can also add values with the plus symbol,

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multiply values with the asterisk symbol,

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and divide by values with the forward slash.

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But here we need the subtraction

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and now adultYears should store the result of age,

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which in this case is 32, minus 18.

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And I will, again, alert this here.

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I will alert adultYears,

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save that, and on my website,

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I therefore get 14 here in this alert box.

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So that's working, and that's important.

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This might sound obvious, and if it is, great,

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but this is a key thing in programming, in general,

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that you can, of course, not just use variables

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to store values so that you can then output them,

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but that you can also work with values

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and do mathematical or other operations

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on them to derive new values

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with which you then might do something,

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like in this case

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outputting that new value again.

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I also want to mention again

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that variables are called variables

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because the values stored in them

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are allowed to change.

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And at the moment that's never the case.

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We always assign a value,

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and then we leave it like that.

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You can also reassign variables.

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You're not limited to that initial value.

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So for example, if the age would change

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for some reason,

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you can again reference age

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and assign a new value

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by adding the equal sign again.

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And then you could store a value of, let's say, 45 here.

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Now the important thing here for that is

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that you don't add the let keyword again, though.

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You don't do that.

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You only add the let keyword

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when you first create and introduce a variable.

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Here, I did already create

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and introduce that variable

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and I'm then just assigning a new value,

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and that is then done without let.

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You only need let if you introduce

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a new variable with a new name.

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So here, for example, we need let.

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We are using age again,

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but only on the right side of the equal sign

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for this calculation.

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The variable, which we create, adultYears,

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is brand new.

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Here for age, on the other hand,

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we are using an existing variable,

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so we don't need let.

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And now that I did change age here,

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what do you think is the result of adultYears now?

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Is it still 14 because it still uses 32

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or will it be 27,

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the result of 45 minus 18?

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Well, let's see.

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If I save that and it reloads, I see 27.

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So it's the result of 45 minus 18.

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And that should make a lot of sense

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because instructions are executed step by step,

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line by line, and therefore we did change age

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before we use it in this calculation,

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and hence the new value of age is used,

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not the old value.

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That wouldn't make too much sense.

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So therefore, in this demo script,

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we now never use the original age of 32.

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We only use the new one.

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And of course, we're still just playing around

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with these features here

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to get a feeling for what we can do

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with JavaScript.