lazy-load.js

'use strict';

const imgs = document.querySelectorAll('[data-src]');

if ('IntersectionObserver' in window) {
    const observer = new IntersectionObserver(entries => {
        for (const entry of entries)
            if (entry.isIntersecting) {
                entry.target.src = entry.target.dataset.src;
                observer.unobserve(entry.target);
            }
    });

    imgs.forEach(e => observer.observe(e));
}
else
    imgs.forEach(e => e.src = e.dataset.src);