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Deep dive into CSS variable

Kishore
Hey guys, I am a javascript developer since 3 yrs now. Love React, React Native. Working as a Front End Developer.
・2 min read

Do you remember the day when you got the first introduction of variable. Yes, I remember properly. I got introduced to variables like others that variable is a bucket where we store any kind of values.

Most of us generally started coding using any functional programming language like Java, Python, C, C++, or even Javascript (Nowadays).

Now coming to the CSS. As of my understanding CSS is technically a programming language because we write CSS programs to accomplish styling for our websites. But a lot of programming concepts were not there in CSS when it was first developed. That's why SASS / SCSS / LESS had come to the picture. One of those missing thing in CSS was variable. But now CSS also supports variable. But syntactically it's a bit different than traditional variable. Let's jump to how variables behave in CSS without much reading.

Declaring variable

This is how a CSS variable declaration looks like

:root {
  --body-bg-color: #f1f1f1;
}

In the above code --body-bg-color is the variable name where #f1f1f1 is the value of it.

Call variable

You have to call a variable like the way below

body {
  background-color: var(--body-bg-color);
}

That's cool.

Rule of thumb

  • The Variable should be declared inside the root selector like :root scope or inside body selector or even inside any selector.
  • Variable name should start with a double hyphen and it's case sensitive.
  • The Variable should call inside var().

Inheritance

You can modify the value of the variable at a later stage when you want to. like the following

:root {
  --text-color: #000;
}
p {
  color: var(--text-color);
}
p.error {
  --text-color: #f00;
  color: var(--text-color);
}

In the above piece of code, we have declared --text-color variable inside :root scope but later when we wanted to modify error style. And then we modified the value inside p.error selector.

Fallback values

You can set some value as a fallback when the variable is not declared.

.cell {
  width: var(--cell-width, 25%);
}

Invalid variable

If the variable value is invalid for the property in that case browser will do 2 things

  1. Check if the property color is inheritable. Yes, but doesn't have any parent with the color property. So move on to the next step.
  2. Set the value to its default initial value, i.e., black.

Example:

HTML

<p>This paragraph is initial black.</p> 

CSS

:root { --text-color: 16px; } 
p { color: blue; } 
p { color: var(--text-color); }

In the above example paragraph text color will not be changed to blue because initially paragraph color is set to color: var(--text-color) but --text-color value is 16px which is not correct for color property.

Example with media queries

Modify variable value inside media queries

:root {
  --cell-width: 25%;
}
.cell {
  width: var(--cell-width);
}
@media only screen and (max-width: 600px) {
  --cell-width: 50%;
  .cell {
    width: var(--cell-width);
  }
}

Conclusion

CSS has evolved a lot now for the past few years. There are a lot of other features that have been introduced to CSS. Stay tuned, will cover all those features in my upcoming posts. Thanks for reading!!

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