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Here’s a CSS variable (formally called a “CSS custom property“):

:root {  --mouse-x: 0px;  --mouse-y: 0px;}

Perhaps you use them to set a position:

.mover {  left: var(--mouse-x);  top: var(--mouse-y);}

To update those values from JavaScript, you’d:

let root = document.documentElement;root.addEventListener("mousemove", e => {  root.style.setProperty('--mouse-x', e.clientX + "px");  root.style.setProperty('--mouse-y', e.clientY + "px");});

That’s all.

See the PenSet CSS Variable with JavaScript by Chris Coyier (@chriscoyier) onCodePen.