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Using CSS Variables

Updated onDec 22, 2021byAlan Morel

#css

Using CSS Variables with JavaScript
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By now, most people know what a CSS preprocessor is and why they are so popular. If not, CSS pre-processors like Stylus work by taking CSS-like markup and converting it to the pure CSS that browsers understand and parse. A major feature common to all CSS pre-processors is the ability to create a variable and use it anywhere you want. This is a powerful feature that allows you to create reusable CSS that can be used across multiple pages.

In other words, instead of doing something like this:

CSS
.thing {
    color: #360a4a;
}

.other-thing {
    color: #360a4a;
}

You can do this in Stylus:

CSS
$sabe-purple: #360a4a;

.thing {
    color: $sabe-purple;
}

.other-thing {
    color: $sabe-purple;
}

CSS pre-processors are great but CSS variables are more powerful. They allow you to create reusable CSS that can be used across multiple pages. They're also native meaning you don't need to use a pre-processor to use them. So how do you use a CSS variable?

Let's look at the above example of how to use CSS variables:

CSS
:root {
    --sabe-purple: #360a4a;
}

.thing {
    color: var(--sabe-purple);
}

.other-thing {
    color: var(--sabe-purple);
}

The end result is the same but the difference is that color is still being set to a variable and not a raw color value, like with the output of Stylus files. This means that you can still modify that variable and let the browser do the work of updating everything else.

Using CSS Variables with JavaScript

Because CSS variables are still variables, we can use JavaScript. to modify their values whenever we want. If you wanted to change the color of .thing and .other-thing, you would need to target both elements and then apply the style changes. With CSS variables, it is as simple as changing the value of that variable.

JAVASCRIPT
document.documentElement.style.setProperty("--sabe-purple", "#472856");

And just like that, we changed the value of --sabe-purple from #360a4a to #472856.

Getting the current value of a CSS variable is also just as simple:

JAVASCRIPT
const value = document.documentElement.style.getPropertyValue("--sabe-purple");

console.log(value);

HTML#472856

Being able to dynamically set and get a CSS variable is an immensely powerful feature because it means that you are no longer held down by one of the biggest downsides of CSS pre-processors: the output in pure CSS is static and cannot be changed without hacking the DOM and manually applying styles all over the place.

If you use a CSS preprocessor variable in 50 different places, you must now change the styles in 50 different places, or be forced to use a class to override them all. Now with CSS variables, you just change the variable's value using a single line of code.

This means that with CSS variables, you now have dynamic styling, meaning styling that isn't static and can change on a fly.

Below is a fully-working example of a pretty simple application of CSS variables. Click on the screen to change the background color to a random color:

HTML
<!DOCTYPE html>
<html>
    <head>
        <title>CSS Variables</title>
        <style>
            :root {
                --color: lightblue;
            }
            body {
                background-color: var(--color);
            }
        </style>
    </head>
    <body>
    </body>
    <script>
        const setDocumentVariable = (propertyName, value) => {
            document.documentElement.style.setProperty(propertyName, value);
        }

        document.addEventListener("click", (event) => {
            setDocumentVariable("--color", "#" + ((1 << 24) * Math.random() | 0).toString(16));
        });
    </script>
</html>

Try out the demo for yourself!

HTML
CSS
JavaScript

CSS variables are pretty fun to play around with and create new experiences with. Definitely keep an eye open for them because they could very well go on to dominate as browser support for them continues to improve.

Resources

Using CSS Variables - MDN Web Docs

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Lesson #13

Display

Learn about the Display CSS property to know how to control how your elements are rendered on the page.

Using CSS Variables

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Using CSS Variables with JavaScript

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