Accessing and modifying CSS variables with Javascript

And why it’s better than SASS variables

Variables in SASS have been around for a while. They allow you to define a variable once at runtime and use that variable in multiple places. Coolness level 3. Coolness level 6? ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓

Interacting with CSS variables with JS post-runtime.

In this demo we are going to build a simple form that will use JS to dynamically update CSS variables(in our case change color and margin of a dot)

jsfiddle result

JSFiddle Demo

Let’s look at the moving pieces here:


css with root vars

Here is some info on CSS :root, here too. From MDN:

The :root CSS pseudo-class matches the root element of a tree representing the document. In HTML, :root represents the <html> element and is identical to the selector html, except that its specificity is higher.

The CSS vars are defined in the :root and applied on lines 13–14. -- is the CSS standard for variable definition.


form markup

Straightforward here, note the attributes on the type="range" input. These hold our min and max values as well as our initial value. Same for type="color" input. The initial color value = #2cba92.


js accessing css vars

Line by line here:

  • 4 — grab footer element
  • 5 — grab NodeList of all inputs on the page
  • 7 — input CHANGE EventListener
  • 8 — input MOUSEMOVE EventListener(updates dynamic margin on slide)
  • 10 — fn to set the value of the desired CSS var(note we add px to the margin var on line 14)

Also from ReasonCode

So that’s it for Accessing and modifying CSS variables with Javascript. Let us know your thoughts and questions and PLEASE follow us on twitter. Keep after it.

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