DEV Community

Cover image for CSS variables: Everything you need to know about
Anjan Shomooder
Anjan Shomooder

Posted on

CSS variables: Everything you need to know about

In this blog, you will learn about css variables also known as custom properties.

You will learn:

  • Why do Css variables exist?
  • How to use it?
  • How cascading works with Css variables?
  • How to access css variables in javascript?
  • How to change css variables with javascript?

For example, You have 5 containers on your webpage. All of the container
the background color will be red.

.container1 {
    background: red;
}

.container2 {
    background: red;
}

.container3 {
    background: red;
}

.container4 {
    background: red;
}

.container5 {
    background: red;
}
Enter fullscreen mode Exit fullscreen mode

Ok. That works fine. But after some time you started to think that the background color should be blue. Now you want to change the color. But now you have to every single container and apply the change. It will work but that is too much work. So,

What is the solution?

What if we can change the color once and that will be applied everywhere?

Yes, we can do that with the help of css variables. Css variables are officially known as custom properties.

So,

How to use Css variables?

We normally define variables at the root.

At the top of your css file select the root like this:

:root {
}
Enter fullscreen mode Exit fullscreen mode

Inside the root you have to declare the variables just like you use properties and value in CSS.

:root {
    --primary-bg-color: red;
}
Enter fullscreen mode Exit fullscreen mode

Syntax

Variable name has to start will --(double dash). For example:

  • --primary-bg-color
  • --secondary-bg-color
  • --primary-font-color

Now you can use those variables as a value in any valid css property. Let's apply it to our example.

:root {
    --primary-bg-color: red;
}

.container1 {
    background: var(--primary-bg-color);
}

.container2 {
    background: var(--primary-bg-color);
}

.container3 {
    background: var(--primary-bg-color);
}

.container4 {
    background: var(--primary-bg-color);
}

.container5 {
    background: var(--primary-bg-color);
}
Enter fullscreen mode Exit fullscreen mode

Now if you want to change the color, just change the value of the variable. And changes will be applied everywhere.

:root {
    --primary-bg-color: blue;
}

.container1 {
    background: var(--primary-bg-color);
}

.container2 {
    background: var(--primary-bg-color);
}

.container3 {
    background: var(--primary-bg-color);
}

.container4 {
    background: var(--primary-bg-color);
}

.container5 {
    background: var(--primary-bg-color);
}
Enter fullscreen mode Exit fullscreen mode

By the way, this blog is originally published on cules coding website.. I would be glad if you give it a visit.

Cules Coding - Everything you need to know about css variables

How cascading works with Css variables?

You can re-declare the value for any css selector. Like this:

:root {
    --primary-bg-color: blue;
}

.container1 {
    background: var(--primary-bg-color);
}

.container2 {
    --primary-bg-color: green;
    background: var(--primary-bg-color);
}

.container3 {
    background: var(--primary-bg-color);
}

.container4 {
    background: var(--primary-bg-color);
}

.container5 {
    background: var(--primary-bg-color);
}
Enter fullscreen mode Exit fullscreen mode

Now the .container2 will have a background color of green. But other containers' background colors will not change. But why?

All of the places that you are using CSS variables are just inheriting values from the root. But when you redeclare the variable for .container2 all of the elements inside the .container2 are just inheriting the value from the .container2.

If the .container2 would look like below, what would be the color of .subcontainer2?

:root {
    --primary-bg-color: blue;
}

.container2 {
    --primary-bg-color: green;
    background: var(--primary-bg-color);
}

.container2 .subcontainer1 {
    background: teal;
}

.container2 .subcontainer1 .subcontainer2 {
    background: var(--primary-bg-color);
}
Enter fullscreen mode Exit fullscreen mode

Will it be green or blue???

It will be green. Why?

Because

  • The CSS parser will check inside .subcontainer2 first. Then it will not find anything.
  • Then it will check subcontainer2's parent .subcontainer1. Again it will find anything.
  • Then it will check .subcontainer1's parent .container. This time the parser will see a variable declaration that has the value green. It will not check anything.

So, this is how it works:

Css variables cascading demonstratition

I know it sounds complex. If you know javascript, it is like scope chain in javscript

How to access CSS variables in javascript?

Yes, you can access css variables in javascript and also you can manipulate them.

const allStyles = getComputedStyle(document.documentElement) // returns all css styles as an object

const bgColor = allStyles.getPropertyValue(--primary - bg - color)

console.log(bgColor) // green or whatever the value is
Enter fullscreen mode Exit fullscreen mode

How to change css variables with javascript?

// Get the root element
const root = document.querySelector(':root')

// changing the value of variable
root.style.setProperty('--primary-bg-color', 'red')
Enter fullscreen mode Exit fullscreen mode

And that's all you need to know about css variables.

Shameless Plug

I have made a video about how to build a carousel postcard with React, Material-UI, and Swiper.js.
If you are interested you can check the video.

You can also demo the application form here

Screenshot of Insta Carousel

Please like and subscribe to Cules Coding. It motivates me to create more content like this.

If you have any questions, please comment down below.
You can reach out to me on social media as @thatanjan.
Stay safe. Goodbye.

About me

Why do I do what I do?

The Internet has revolutionized our life. I want to make the internet more beautiful and useful.

What do I do?

I ended up being a full-stack software engineer.

What can I do?

I can develop complex full-stack web applications like social media applications or e-commerce sites.

What have I done?

I have developed a social media application called Confession. The goal of this application is to help people overcome their imposter syndrome by sharing our failure stories.
Alt Text

I also love to share my knowledge. So, I run a youtube channel called Cules Coding where I teach people full-stack web development, data structure algorithms, and many more. So, Subscribe to Cules Coding so that you don't miss the cool stuff.

Want to work with me?

I am looking for a team where I can show my ambition and passion and produce great value for them.
Contact me through my email or any social media as @thatanjan. I would be happy to have a touch with you.

Contacts

Blogs you might want to read:

Videos might you might want to watch:




Discussion (0)