owntone-server/docs/assets/extra.js

/*
 * Add click event handler for images with class="zoom"
 */
document.querySelectorAll('img.zoom').forEach(item => {
    const p = item.parentElement;
    if (!p.classList.contains('processed')) {
        p.classList.add('processed');
        if (p.querySelectorAll('img.zoom').length === p.children.length) {
            p.classList.add('zoom-wrapper');
        }
    }
    item.addEventListener('click', function () {
        const img = document.getElementById('fullscreen-image-img');
        img.setAttribute('src', this.getAttribute('src'));
        img.setAttribute('alt', this.getAttribute('alt'));

        const div = document.getElementById('fullscreen-image');
        div.classList.replace('hidden', 'visible');
    })
});

var div = document.createElement('div');
div.classList.add('fullscreen-image-background', 'hidden');
div.id = 'fullscreen-image';
var img = document.createElement('img');
img.id = 'fullscreen-image-img';
div.appendChild(img);

div.addEventListener('click', function () {
    this.classList.replace('visible', 'hidden');
});
document.body.appendChild(div);