Inhalte nur zu laden, wenn sie wirklich benötigt werden, sorgt nicht nur dafür, dass die Seite schneller das Dom-Ready erreicht, sondern erspart unter Umständen jede Menge Datentransfer, was nicht nur Google erfreut, sondern auch den Betrachter der Website.
Diese Implementierung bringt eine Filterung aller Bilder auf der Seite mit sich, die im data-src-Attribut einen Inhalt entalten, dieser Wert braucht nur in das src-Attribut verschoben werden, sobald das Bild im sichtbaren Bereich auftaucht und auch wirklich sichtbar sein soll.
Einzige Voraussetzung <img data-src="..." loading="lazy" />
Diese Implementierung unterstützt die native Variante loading="lazy" als img Attribute bei einem aktuellen Browser (ab Opera 64, Firefox 75, Chrome 76, Edge 79, Andoid Browser 80, Chrome for Android 80), aber auch den IntersectionObserver bei den etwas äteren (ab Opera 45, Firefox 55, Chrome 58, Edge 16, Safari 12.2, Andoid Browser 80, Chrome for Android 80, ...), sowie eine Variante über EventLisener für die ganz alten Browser wie z.B. den Internet Explorer, dadurch entsteht auch die breiteste Abdeckung für Lazy Loading. Zusätzlich wurde das Laden der Bilder vor dem Ausdrucken mit eingebaut, welches bisher von keinem Browser als Standard zur Verfügung gestellt wird.
// coding: utf-8
/** Created by: Udo Schmal | https://www.gocher.me/ */
(function() {
'use strict';
// lazy loading image / iframe
function loadSrc(el) {
if (el.getAttribute('data-src')) {
el.setAttribute('src', el.getAttribute('data-src'));
el.removeAttribute('data-src');
}
if (el.getAttribute('data-srcset')) {
el.setAttribute('srcset', el.getAttribute('data-srcset'));
el.removeAttribute('data-srcset');
}
if (el.getAttribute('data-sizes')) {
el.setAttribute('sizes', el.getAttribute('data-sizes'));
el.removeAttribute('data-sizes');
}
return true;
}
// lazy preloading video
function preloadVideo(el) {
if (el.getAttribute('data-autoplay')) {
el.setAttribute('preload', 'auto');
el.setAttribute('autoplay', 'autoplay');
el.autoplay = true;
el.play();
el.removeAttribute('data-autoplay');
}
}
// Viewport prototype to trigger event if element moves into viewport the fist time
function Viewport(els, callback) {
this.els = els;
this.callback = callback;
this.active = false;
var self = this;
var hidden = [];
function alreadyObserved(el) {
for(var i=0; i<hidden.length; i++) {
if (el === hidden[i]) {
return true;
}
}
return false;
}
for (var i=0; i < els.length; i++) {
var el = els[i];
if (!el.parentNode.style.position) {
// parent must have position attribute for getBoundingClientRect
el.parentNode.style.position = 'relative';
while(el && el.tagName.toLowerCase() !== 'body') {
if (window.getComputedStyle(el).display === "none") {
if (!alreadyObserved(el)) {
hidden.push(el);
var observer = new MutationObserver(function(mutations) {
if (mutations[0].target.style.display !== 'none') {
self.handleEvent();
}
});
observer.observe(el, { attributes: true });
break;
}
}
el = el.parentNode;
}
}
}
this.handleEvent.bind(this);
// is already in viewport after dom ready
function ready(f){
/complete|loaded/i.test(document.readyState) ? f() : setTimeout(function(){ready(f);},9);
}
ready(function(){self.handleEvent();});
// add event listener to scroll event to check visibility change
document.addEventListener('scroll', this, true);
window.addEventListener('resize', this, true);
window.addEventListener('orientationchange', this, true);
}
Viewport.prototype = {
// check if element is visible
isVisible: function (el) {
var style = window.getComputedStyle(el);
return !!(el.offsetWidth || el.offsetHeight || el.getClientRects().length);
},
// check if element is in viewport
isInViewport: function (el) {
var bounding = el.getBoundingClientRect();
return (
bounding.bottom >= 0 &&
bounding.right >= 0 &&
bounding.top <= (window.innerHeight || document.documentElement.clientHeight) &&
bounding.left <= (window.innerWidth || document.documentElement.clientWidth)
);
},
// handle the visibility check and the resulting action
handleEvent: function () {
if (this.active === false) {
this.active = true;
for (var i=0; i < this.els.length; i++) {
if (this.isInViewport(this.els[i]) && this.isVisible(this.els[i])) {
this.callback(this.els[i]);
}
}
this.active = false;
}
}
};
const type = { event: 0, intersection: 1, loading: 2};
var support;
// if Google Chrome for Android "reduce data usage" / "lite mode" is active
// var dataSave = !('connection' in navigator) || (navigator.connection.saveData);
// if Google Chrome chrome://flags/#enable-lazy-image-loading is enabled
if ("loading" in HTMLImageElement.prototype) {
console.log('native lazy loading');
/* Opera 64, Firefox 75, Chrome 76, Edge 79, Andoid Browser 80, Chrome for Android 80 */
support = type.loading;
} else if ("IntersectionObserver" in window) {
console.log('IntersectionObserver lazy loading');
/* Opera 45, Firefox 55, Chrome 58, Edge 16, Safari 12.2, Andoid Browser 80, Chrome for Android 80, ... */
support = type.intersection;
} else {
console.log('Event based lazy loading');
/* Internet Explorer and other old browsers */
support = type.event;
}
// activate viewport for image loading
var images = document.querySelectorAll("img[data-src]");
if (images.length > 0) {
// load all images before print
window.addEventListener("beforeprint", function(event) {
for (var i=0; i < images.length; i++) {
if (support == type.loading) {
images[i].setAttribute('loading', 'eager');
} else {
loadSrc(images[i]);
}
}
function sleep(milliseconds) {
const date = Date.now();
let currentDate = null;
do {
currentDate = Date.now();
} while (currentDate - date < milliseconds);
}
sleep(1000);
});
// use native lazy loading
if (support == type.loading) {
// set attribute loading to lazy and move attribute value from data-src to src
for (var i=0; i < images.length; i++) {
loadSrc(images[i]);
}
} else if (support == type.intersection) {
// create intersection observer
let lazyobserver = new IntersectionObserver(function(entries, observer) {
for (var i=0; i < entries.length; i++) {
if (entries[i].isIntersecting) {
loadSrc(entries[i].target);
lazyobserver.unobserve(entries[i].target);
}
}
},
{
threshold: [0.1],
// Set a minimum delay between notifications
delay: 100
});
// start observing
for (var i=0; i < images.length; i++) {
lazyobserver.observe(images[i]);
}
} else /* if (support == type.event) */ {
// use eventListeners
new Viewport(images, loadSrc);
}
}
// activate viewport for iframe loading
var iframes = document.querySelectorAll("iframe[data-src]");
if (iframes.length > 0) {
if (support == type.loading) {
// set attribute loading to lazy and move attribute value from data-src to src
for (var i=0; i < iframes.length; i++) {
loadSrc(iframes[i]);
}
} else if (support == type.intersection) {
// create intersection observer
let lazyiframeobserver = new IntersectionObserver(function(entries, observer) {
for (var i=0; i < entries.length; i++) {
if (entries[i].isIntersecting) {
loadSrc(entries[i].target);
lazyiframeobserver.unobserve(entries[i].target);
}
}
},
{
threshold: [0.1],
// Set a minimum delay between notifications
delay: 100
});
// start observing
for (var i=0; i < iframes.length; i++) {
lazyiframeobserver.observe(iframes[i]);
}
} else {
new Viewport(iframes, loadSrc);
}
}
// activate viewport for video preloading
var videos = document.querySelectorAll("video[data-autoplay]");
if (videos.length > 0) {
new Viewport(videos, preloadVideo);
}
})();
-
Images
-
Videos
-
IFrame (GoogleMaps) external
-
IFrame (GoogleMaps)
-
YouTube external
-
YouTube (autostart)
-
Vimeo external
-
Vimeo popup
-
X (Twitter)