resize-observer.js

// Only run if ResizeObserver is supported.
if ('ResizeObserver' in self) {
  // Create a single ResizeObserver instance to handle all
  // container elements. The instance is created with a callback,
  // which is invoked as soon as an element is observed as well
  // as any time that element's size changes.
  var ro = new ResizeObserver(function(entries) {
    // Default breakpoints that should apply to all observed
    // elements that don't define their own custom breakpoints.
    var breakpoints = {tabletUp: 384, MD: 576, LG: 768, XL: 960};

    entries.forEach(function(entry) {
      // Update the matching breakpoints on the observed element.
      Object.keys(breakpoints).forEach(function(breakpoint) {
        var minWidth = breakpoints[breakpoint];
        if (entry.contentRect.width >= minWidth) {
          entry.target.classList.add(breakpoint);
        } else {
          entry.target.classList.remove(breakpoint);
        }
      });
    });
  });

  // Find all elements with the `data-observe-resizes` attribute
  // and start observing them.
  var elements = document.querySelectorAll('[data-observe-resizes]');
  for (var element, i = 0; element = elements[i]; i++) {
    ro.observe(element);
  }
}