lazyimgs.js

document.addEventListener("DOMContentLoaded", function() {
  var lazyImages = [].slice.call(document.getElementsByClassName("js-lazy-img"));

  if ("IntersectionObserver" in window) {
    var config = {
      // If the image gets within 250px of the browser's viewport, start the download:
      rootMargin: '250px 0px',
    };

    var lazyImageObserver = new IntersectionObserver(function(entries, observer) {
      entries.forEach(function(entry) {
        if (entry.isIntersecting) {
          var lazyImage = entry.target;
          lazyImage.src = lazyImage.dataset.src;
          // Not all images are responsive, sometimes we always serve the original
          if (lazyImage.dataset.srcset) {
            lazyImage.srcset = lazyImage.dataset.srcset;
          }
          lazyImageObserver.unobserve(lazyImage);
        }
      });
    }, config);

    lazyImages.forEach(function(lazyImage) {
      lazyImageObserver.observe(lazyImage);
    });
  }
  else {
    // For browsers that don't support IntersectionObserver yet, load all the images now:
    lazyImages.forEach(function(lazyImage) {
      lazyImage.src = lazyImage.dataset.src;
      // Not all images are responsive, sometimes we always serve the original
      if (lazyImage.dataset.srcset) {
        lazyImage.srcset = lazyImage.dataset.srcset;
      }
    });
  }
});

Can this code ever run on the server? If so, we'll need to make the IntersectionObserver more robust, e.g.
typeof window !== 'undefined' && "IntersectionObserver" in window

.lazy seems like a dangerously short and generic CSS classname to hang such complex behaviour upon. I suggest using something like .js-lazy, at the very least, to make it clear it's a CSS class that serves as a JS signal. The alternative is to use a data attribute instead, since you're already relying on them for storing the URL data. That would make the query look like e.g. img[data-lazy].

1- No, this should never be running on the server. It needs a view port, and that is entirely client dependent.

2- Fair point on the class name, I'll expand it to something more unique.