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CSS variables weren't created just to offer the same functionality as preprocessor variables. Variables are a game changer in CSS, and there are plenty of differences and advantages to using CSS variables over preprocessor variables.
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CSS variables weren't created
just to be the CSS equivalent
0:00
of preprocessor variables, or
even offer the same functionality as them.
0:04
Variables in CSS can do many things you
can't do with preprocessor variables.
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They each behave differently
under certain circumstances.
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In fact, it's like they're two different
entities and live in separate worlds.
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Let's look at why.
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You see,
preprocessors don't run in the browser.
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The variables you declare living
a source file in your machine.
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Preprocessors just replace variables
with static values when they're
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compiled to CSS.
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The browser knows nothing
about the variables and
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the variables themselves know
nothing about your markup.
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On the other hand, CSS variables are
entirely more flexible than compiled code.
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Because they live in the browser.
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When you update a CSS variable's value,
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the browser updates it anywhere
it's referenced at runtime.
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This gives them dynamic capabilities and
opens up lots of interesting possibilities
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than just replacing colors and
font values.
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For example, in a preprocessor you can't
update the value of a variable from within
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a media query.
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So in this SAAS example, the headline
classes margin-bottom value cannot be
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changed to 2rem inside a media query by
updating the gutter variable like this.
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But you can with CSS variables.
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Their values are updated
live inside media queries.
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For instance,
in the latest style sheet for
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this video, I see that the gutter variable
is scoped to the root element, so
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I'm going to change its value
from 12 pixels to 1rem.
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Then I'll scroll down and give the header
a margin-bottom property that reference
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the gutter variable as its value.
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Also the cards on the page are being
laid out with CSS grid via
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the main content div.
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So I'll add a gutter or
gap between the cards.
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First I'll target
the class main-content and
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add a grid-gap property with its
value set to the gutter variable.
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Now I'll use media queries to provide
a new context that updates the value of
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the gutter variable without having to
respecify any selectors like header and
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main content, or
any CSS properties inside the media query.
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So first I'll create a new media query.
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That target devices that
are a minimum of 576 pixels.
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And here I'll update the value
of gutter in the root scope.
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So I'll add the root pseudo-class.
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And inside that,
lets update the value of gutter to 3rem.
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Then in the next media query,
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I'll once again update the value
of gutter inside root.
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So I'll simply copy the selector
I wrote in the first media query.
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Paste it here.
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And this time I'll set it to
a larger value like 5rem, just so
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we can see the changes in the browser.
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So notice how I'm only including the root
selector scope and the updated variables.
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Not the main content and
hetero selectors, or
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the actual CSS properties
that reference the variable.
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I'll give this a save.
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And as you'll see, the gutter variable
is responsive and updates dynamically.
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As I resize the browser,
the margin values change.
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Now have a look at this
fun demo I created,
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that shows how CSS variables can
reset values inside media queries.
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This time I'm using a CSS
animation of a ball.
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As you can see, the ball moves up and down
on small screens, then switches to a side
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to side animation on medium screens, then
a diagonal animation on large screens.
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In the CSS for this, the ball rule
has a variable called animate,
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which calls the bounce
keyframe sequence by default.
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In the first media query,
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the animate variable is set to
the slide keyframe sequence,
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then it's set to the diagonal sequence
in this wider media query context.
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You'll find a link to this demo in
the teacher's notes of this video.
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You can even forward
the workspace snapshot and
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experiment with the values yourself.
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Another example of how CSS variables
can be updated and altered dynamically,
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is updating their values inline within
the style attribute of an element.
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For example, the learn more buttons
in the cards use the color primary
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variable to set their background color.
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I'll open up index.html and
add a style attribute to one
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of the anchor elements
with the btn-info class.
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And here I'll update the value
of the color primary variable,
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to let's say, firebrick.
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And as you can see, we immediately
see the change in the browser.
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This is not possible with
static preprocessor variables,
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because, as I mentioned earlier,
they know nothing about your markup.
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All right, so in the next video,
you'll learn how CSS variables cascade and
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inherit their value from their parent.
5:21
We'll define and reset variables at
different levels of specificity.
5:23
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