26.02.2026 | Holger Weischenberg

Using CSS media queries and CSS variables in JavaScript

cssMediaQuery.js
// Retrieve all computed styles from the root HTML element (:root)
const rootStyles = getComputedStyle(document.documentElement);

// Get the value of the CSS variable --Breakpoint.
// Use a fallback (e.g., 768px) if the variable is not defined.
const breakpoint = rootStyles.getPropertyValue('--Breakpoint').trim() || '768px';

// Create a MediaQueryList based on the breakpoint value
const mediaQuery = window.matchMedia(`(min-width: ${breakpoint})`);

// Function that runs when the viewport is at or above the breakpoint
function handleDesktopLayout() {
    console.log("Viewport is at or above the breakpoint.");
}

// Function that runs when the viewport drops below the breakpoint
function handleMobileLayout() {
    console.log("Viewport is below the breakpoint.");
}

// Early return for the initial state:
// If the viewport does NOT meet the breakpoint requirement, run mobile logic and exit
if (!mediaQuery.matches) {
    handleMobileLayout();
    // No desktop logic needed at this point
} else {
    handleDesktopLayout();
}

// Listen for changes in viewport size relative to the breakpoint
mediaQuery.addEventListener('change', (event) => {
    if (event.matches) {
        // The viewport just became equal to or larger than the breakpoint
        handleDesktopLayout();
    } else {
        // The viewport just became smaller than the breakpoint
        handleMobileLayout();
    }
});

Issue:

You want JavaScript to execute something based on a breakpoint in a CSS media query.

Solution:

Read the breakpoint value directly from the --Breakpoint CSS variable, create a matchMedia query from it, and use an event listener to run JavaScript whenever the viewport crosses that breakpoint.

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