The easiest way to get and set CSS Variables in JavaScript

Published:Dec 5, 2023

CSS Variables have become super popular for many reasons. One of them these reasons is — you can now manipulate CSS easily with JavaScript.

In this article, we’ll go through how to get and set CSS variables using the methods Vanilla JavaScript provides us with, then we’ll go through how to make the process easier with simple helper functions.

Let’s start from the beginning.

Using CSS Variables

CSS Variables can be written on two kinds of selectors:

The :root selector
Any other CSS selector

:root {
  --variable: #111;
}

.element {
  --variable: #222;
}

This will be an important distinction later because how you access the CSS variables depends on whether you use it on the :root selector or other selectors.

Getting CSS Variables with JavaScript

You can use getComputedStyle and getPropertyValue to get the CSS Variable in JavaScript. Here’s the syntax:

getComputedStyle(element).getPropertyValue('--variable')

element must be replaced by the HTML Element that contains the CSS Variable.

If you wrote the variable in :root, you can use document.documentElement or document.body
If you wrote the variable in another selector, you need to use document.querySelector to find the element first.

// Getting a CSS variable from :root
const value = getComputedStyle(document.body).getPropertyValue('--variable') // #111

// Getting a CSS variable from an HTML element with the `.element` class
const element = document.querySelector('.element')
const value = getComputedStyle(element).getPropertyValue('--variable')
console.log(value) // #222

Setting CSS Variables with JavaScript

You can set CSS Variables with the following syntax:

element.style.setProperty(property, value)

element must be the HTML element you’re trying to set the CSS variable to.
property is the CSS variable you want to set. You must include the double dashes (--) that signify CSS variables here.
value is the value you want to set.

The same thing applies as before:

If you want to set CSS variables on the :root element, you can use document.body or document.documentElement as element.
If you want to set CSS variables on any other element, you must get that element first.

// Setting CSS Variables on :root
document.body.style.setProperty('--variable', 'blue')

// Setting CSS Variables on an HTML Element with the `.element` class
const element = document.querySelector('.element')
element.style.setProperty('--variable', 'blue')

Helper functions that make it easier to get and set CSS variables

The above processes are simple, but it can be tedious remember the correct way to get and set CSS variables.

To make the process easier for me, I created two helper functions to help me out:

getCSSVar — Gets CSS variable from an element
setCSSVar — Sets CSS variable on an element

You can install these two helpers by installing Splendid UI, then import them from the utils subpath.

npm install splendid-ui --save

import { getCSSVar, setCSSVar } from 'splendid-ui/svelte'

Let’s go through how to use each of these helper functions.

get css var

getCSSVar helps you get a CSS variable from an element.

const value = getCSSVar(element, property)

element is the element you want to get the CSS property from.
property is the name of the CSS variable.

getCSSVar makes things a little easier if you have to get a CSS Variable from an element — you can just pass in the selector and it will do a querySelector for you.

// Getting CSS variable from an HTML element with the `element` class.
const value = getCSSVar('.element', '--variable')
console.log(value) // #222

You can still pass in an HTML element and getCSSVar will respect that element you passed in.

// This works too
const element = document.querySelector('.element')
const value = getCSSVar(element, '--variable')
console.log(value) // #222

If the CSS variable is on the :root selector, you can just use document.documentElement or document.body as the element.

// Getting CSS variable from the :root selector
const value = getCSSVar(document.body, '--variable')
console.log(value) // #111

set css var

setCSSVar lets you set a CSS variable on an element. The syntax is as follows:

setCSSVar(element, property, value)

element is the element you want to set the CSS property on.
property is the name of the CSS variable.
value is the value you want to set.

The same rules for element in getCSSVar apply for setCSSVar.

You can pass in a selector into setCSSVar and it will find the element for you.

// Setting CSS variable on an element with the `.element` class
setCSSVar('.element', '--variable')

setCSSVar will respect the HTML element you pass into element too.

// This works too
const element = document.querySelector('.element')
setCSSVar(element, '--variable', 'blue')

If the CSS variable is on the :root selector, you can just use document.documentElement or document.body as the element.

// Setting CSS variable on the :root selector
setCSSVar(document.body, '--variable')

That’s it!

Wrapping Up

CSS variables are powerful but working with CSS variables in JavaScript can become tedious — especially if you need to remember how different it is to get and set CSS Variables.

To make the process easier, I created getCSSVar and setCSSVar helper functions in Splendid UI.

Take them out for a spin and let me know what you think!

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