Original subtitles

Now that we're getting

more comfortable with JavaScript,

it's time to learn about a more theoretical concept.

And that's the difference

between statements and expressions.

And this is gonna be just a very high level overview,

just so that the future lecturers make more sense.

So essentially, an expression is a piece of code

that produces a value.

For example, three plus four is an expression

because this is gonna produce a value, okay?

So I don't even mean the entire line of code.

What I mean is really just this three plus four.

This is an expression, okay?

And again, because it does produce a value.

The same goes, for example for just any number.

This for example is an expression too,

even though it's just a simple value.

But it's still also an expression,

because again, this itself

will produce a value in JavaScript.

Or we can do this.

So true and false and,

not false.

So all of this here,

is an expression, right?

Because this will in the end, produce some Boolean value.

Now, on the other hand, we have statements.

And the statement is like a bigger piece of code

that is executed

and which does not produce a value on itself.

And we can compare this with normal spoken language.

And so in this example,

a declaration is like a complete sentence

and expressions are like the words

that make up the sentences.

Now, basically we write our whole programs

as a sequence of actions.

And these actions are statements.

So let's take as an example, the, if else, statement.

So let's say,

if 23 is greater than 10,

then, well just do something.

So create a string called, 23 is bigger.

The code itself doesn't really matter here.

So this, if else statement is in fact a statement.

And so that's the reason why I have called it

the if else statement all the time.

And the same is true for the switch statement.

So this statement here

doesn't really produce a value, does it?

All it does is, in this case,

it simply declares is variable called STR.

But that's it.

It performs some actions,

in this case, declaring this variable,

but it doesn't produce a value.

And so in that, it's different from,

for example, having this here, okay?

Now the string itself, again is an expression.

So this is probably a bit confusing,

but that's not a problem, okay?

My goal here is not that you understand

the exact rules of what an expression is

and what a statement is,

because, in fact, it's all a bit fuzzy.

So my main goal with this lecture is really

that you know that statements and expressions exist,

and that expressions produce values,

and that statements are like full sentences

that translate our actions.

So the actions that we want the program to perform, okay?

So really that's all you need to know,

and all you need to basically memorize from this video.

We don't need to go into all the details.

So there's no need to start asking a bunch of questions

about this in the Q and A section, all right?

So this will all make even more sense as we move on.

So I said that this string here is itself an expression.

And then this whole line of code is a statement,

because this again does not produce a value.

And so basically, whenever something ends with a semicolon,

that's then a statement.

It's like a complete sentence, okay?

Now this difference between expressions and statements

is important to know

because JavaScript expects

statements and expressions in different places.

For example, in a template literal,

we can only insert expressions, but not statements.

So let me just very quickly

demonstrate that to you.

So we can say, I am,

and then our placeholder here, basically.

And so in here, we need to put an expression.

So something that produces a value

which can then be put into the string

that we're building here using the template literal.

So let's say,

just to do some calculations here.

So 2037 minus 1991,

years old.

So again, this is an expression here.

And so this one will work.

However, we cannot do this.

For example, we could not insert

this if statement in here.

It wouldn't make any sense at all, okay?

Let's try that.

And so we actually get right away,

unexpected token, if.

And that's because JavaScript knows that this is a statement

and statements don't make sense

where JavaScript expects an expression.

Now, if we had a variable, for example,

then if we used that here,

then this would actually also be an expression.

Because this variable

will essentially just be replaced with this string.

And so this produces a value,

and so of course this is completely acceptable.

Okay, and that's actually all I had to show you

in this video.

Again, my goal was really just to make sure

that you kind of understand

the difference between expressions and declarations

so that moving forward,

we are really both on the same page here.