SOURCE

const images = document.querySelectorAll('img');
const observer = new IntersectionObserver(entries =>{
    entries.forEach(entry => {
        if(entry.isIntersecting) {
            const img = entry.target;
            const data_src = img.getAttribute('data-src');
            img.setAttribure('src', data_src);
            observer.unobserve(img);
        }
    })
})

images.forEach(image => {
    observer.observe(image)
})