Updating a CSS Variable with JavaScript

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 Pen Set CSS Variable with JavaScript by Chris Coyier (@chriscoyier) on CodePen.

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