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Since JSX is a syntax extension to the JavaScript language, it accepts any valid JavaScript expressions written inside curly braces. This allows you to make your JSX more dynamic.
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Let's dive deeper into JSX and
0:00
explore three important rules
to follow when working with it.
0:02
The first rule we'll cover is JSX
must return a single root element.
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Let me show you what they mean by this.
0:12
Applications usually consist
of more than just a heading.
0:14
So let's add a p tag
under our heading tag.
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To write our JSX across multiple lines,
I'll wrap our h1 tag in parentheses.
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Inside the p tag,
I'll write I just learned how to
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create a React node and
render it into the DOM.
0:39
We can see we're getting an error, JSX
expressions must have one parent element.
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To fix this error, we can wrap our
JSX tags in a div tag, like so.
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Now, when we take a look at the browser,
we can see the p tag we added.
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In the DevTools, we can see the div
tags wrapped around the h1 and p tags.
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If we don't want to add a div tag
just to return both the h1 and
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p tags,
instead we could use React's Fragment tag.
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Fragment lets you group elements
without a wrapper node.
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To use React's Fragment,
we'll just wrap our h1 and
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p tag in an empty tag like so.
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In the browser,
the page looks exactly the same, but
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when we take a look at the DevTools,
we no longer see the h1 and
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p tags wrapped in the extra div tag.
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So why do multiple JSX
tags need to be wrapped?
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React's Docs answers this question, and
you can read their answer on the screen.
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But it basically says, when you
write multiple JSX tags, like h1 and
2:00
p tags, React needs to transform each
of these tags into a JavaScript object.
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However, since you can't return
multiple objects directly,
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you have to wrap those JSX
tags inside something else,
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like a parent tag or React's Fragment.
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This way, we're only returning one
JavaScript object instead of two.
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The second rule is to close all tags.
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In JSX, all tags must be closed,
even if they don't have any children.
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Unlike in HTML,
where some tags can be self-closing,
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in JSX, you need to include
the closing tag for every element.
2:45
For example,
if we create an image tag in HTML,
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we would just write img src= image.jpg.
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But we get an error, JSX element img
has no corresponding closing tag.
3:02
instead in JSX,
we should write image tags like this.
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Notice the closing slash
at the end of the tag.
3:13
The third rule is to
camelCase most of the things.
3:18
In HTML, if you wanted to add
a class attribute to the p tag,
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you would just add class
inside the p tag like so.
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But JSX turns into JavaScript,
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so you can't use reserved
words like class or for.
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You also can't use names
that contain dashes.
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Since class is a reserved word, in React,
you would write className instead.
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You can find all of these
attributes in the list of
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DOM component props in
the React documentation.
3:55
But if you get one wrong, don't worry,
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React will print a message with
a possible correction to the console.
4:03
The error message we see is
Warning: Invalid DOM property class.
4:07
Did you mean className?
4:12
In the Elements tab, we can see it's
still added the class attribute.
4:14
But let's go back to our code and
change class to className.
4:19
Now that we covered the three rules,
let's move on to embedding
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JavaScript expressions in
JSX using curly braces.
4:30
Since JSX is an extension
to the JavaScript language,
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it accepts any valid JavaScript
expressions written inside curly braces.
4:39
This allows you to make
your JSX more dynamic.
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For example, I'll add two string
4:49
variables that will contain
the values of the h1 and p tags.
4:52
Then we can use the title and
description variables inside the h1 and
5:05
p tags by wrapping them in curly braces.
5:11
If we check our app in the browser, you
can see that everything is still the same.
5:14
When you use curly braces in JSX,
it's called a JSX expression.
5:20
A JSX expression is always
surrounded by curly braces and
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can be placed between JSX tags like this,
or
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as the value of an attribute in a JSX tag.
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For example, I'll create a new variable
5:40
named myTitleId and set it to a string.
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And now I can use the value assigned to
myTitleId as the h1's id value like so.
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If we inspect the h1 tag, we see that
the id attribute is set to main-title.
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Good, so now let's make our
static text just a little
6:07
bit more dynamic with a JSX expression.
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Let's create the variable name and
assign our name as a string.
6:15
Inside the h1 tag, I'll replace the title
variable with the name variable,
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then add an 's after it and
the text First React Element.
6:29
I'll go ahead and
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delete the title variable up top
since we're no longer using it.
6:37
Let's save our file, and
over in the browser,
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we see the text
Laura's First React Element as the title.
6:45
Just remember that when you're
including the curly braces,
6:50
you're exiting JSX and
entering JavaScript territory.
6:54
The curly braces lets JSX know
that you're writing JavaScript.
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For example, you can do math operations.
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I'll create an input element and
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set the value to a JSX expression
that multiplies 10 by 20.
7:10
And over in the browser,
we see 200 as the input field's value.
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There are lots of other things
you can do with JSX expressions,
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which we'll learn later in this course.
7:26
But feel free to try writing
a few expressions on your own
7:29
after this video to explore
the capabilities of JSX.
7:34
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