loading-attribute-polyfill.js

/*
 * Loading attribute polyfill - https://github.com/mfranzke/loading-attribute-polyfill
 * @license Copyright(c) 2019 by Maximilian Franzke
 * Credits for the initial kickstarter / script to @Sora2455, and supported by @cbirdsong, @eklingen, @DaPo, @nextgenthemes, @diogoterremoto, @dracos and @Flimm - many thanks for that !
 */
/*
 * A minimal and dependency-free vanilla JavaScript loading attribute polyfill.
 * Supports standard's functionality and tests for native support upfront.
 * Elsewhere the functionality gets emulated with the support of noscript wrapper tags.
 * Use an IntersectionObserver polyfill in case of IE11 support necessary.
 */

(function(noscriptClass, rootMargin) {
	'use strict';

	var config = {
		// Start download if the item gets within 256px in the Y axis
		rootMargin: rootMargin || '0px 0px 256px 0px',
		threshold: 0.01,
		lazyImage: 'img[loading="lazy"]',
		lazyIframe: 'iframe[loading="lazy"]'
	};

	// Device/browser capabilities object
	var capabilities = {
		loading:
			'loading' in HTMLImageElement.prototype &&
			'loading' in HTMLIFrameElement.prototype,
		scrolling: 'onscroll' in window
	};

	// Nodelist foreach polyfill / source: https://stackoverflow.com/a/46929259
	if (
		typeof NodeList !== 'undefined' &&
		NodeList.prototype &&
		!NodeList.prototype.forEach
	) {
		// Yes, there's really no need for `Object.defineProperty` here
		NodeList.prototype.forEach = Array.prototype.forEach;
	}

	// Define according to browsers support of the IntersectionObserver feature (missing e.g. on IE11 or Safari 11)
	var intersectionObserver;

	if ('IntersectionObserver' in window) {
		intersectionObserver = new IntersectionObserver(onIntersection, config);
	}

	// On using a browser w/o requestAnimationFrame support (IE9, Opera Mini), just run the passed function
	var rAFWrapper;

	if ('requestAnimationFrame' in window) {
		rAFWrapper = window.requestAnimationFrame;
	} else {
		rAFWrapper = function(func) {
			func();
		};
	}

	var temporaryImage =
		'data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7';

	/**
	 * Put the source and srcset back where it belongs - now that the elements content is attached to the document, it will load now
	 * @param {Object} lazyItem Current item to be restored after lazy loading.
	 */
	function restoreSource(lazyItem) {
		var srcsetItems = [];

		// Just in case the img is the decendent of a picture element, check for source tags
		if (lazyItem.parentNode.tagName.toLowerCase() === 'picture') {
			removePlaceholderSource(lazyItem.parentNode);

			srcsetItems = Array.prototype.slice.call(
				lazyItem.parentNode.querySelectorAll('source')
			);
		}

		srcsetItems.push(lazyItem);

		srcsetItems.forEach(function(item) {
			if (item.dataset.lazySrcset) {
				item.setAttribute('srcset', item.dataset.lazySrcset);
				delete item.dataset.lazySrcset;
			}
		});

		lazyItem.setAttribute('src', lazyItem.dataset.lazySrc);
		delete lazyItem.dataset.lazySrc;
	}

	/**
	 * Remove the source tag preventing the loading of picture assets
	 * @param {Object} lazyItemPicture Current <picture> item to be restored after lazy loading.
	 */
	function removePlaceholderSource(lazyItemPicture) {
		var placeholderSource = lazyItemPicture.querySelector(
			'source[data-lazy-remove]'
		);

		if (placeholderSource) {
			lazyItemPicture.removeChild(placeholderSource); // Preferred .removeChild over .remove here for IE
		}
	}

	/**
	 * Handle IntersectionObservers callback
	 * @param {Object} entries Target elements Intersection observed changes
	 * @param {Object} observer IntersectionObserver instance reference
	 */
	function onIntersection(entries, observer) {
		entries.forEach(function(entry) {
			// Mitigation for EDGE lacking support of .isIntersecting until v15, compare to e.g. https://github.com/w3c/IntersectionObserver/issues/211#issuecomment-309144669
			if (entry.intersectionRatio === 0) {
				return;
			}

			// If the item is visible now, load it and stop watching it
			var lazyItem = entry.target;

			observer.unobserve(lazyItem);

			restoreSource(lazyItem);
		});
	}

	/**
	 * Handle printing the page
	 */
	function onPrinting() {
		if (typeof window.matchMedia === 'undefined') {
			return;
		}

		var mediaQueryList = window.matchMedia('print');

		mediaQueryList.addListener(function(mql) {
			if (mql.matches) {
				document
					.querySelectorAll(
						config.lazyImage +
							'[data-lazy-src],' +
							config.lazyIframe +
							'[data-lazy-src]'
					)
					.forEach(function(lazyItem) {
						restoreSource(lazyItem);
					});
			}
		});
	}

	/**
	 * Get and prepare the HTML code depending on feature detection for both image as well as iframe,
	 * and if not scrolling supported, because it's a Google or Bing Bot
	 * @param {String} lazyAreaHtml Noscript inner HTML code that src-urls need to get rewritten
	 */
	function getAndPrepareHTMLCode(noScriptTag) {
		// The contents of a <noscript> tag are treated as text to JavaScript
		var lazyAreaHtml = noScriptTag.textContent || noScriptTag.innerHTML;

		if (
			!capabilities.loading &&
			capabilities.scrolling &&
			typeof document.documentElement.dataset !== 'undefined'
		) {
			// Check for IntersectionObserver support
			if (typeof intersectionObserver === 'undefined') {
				// Attach abandonned attribute 'lazyload' to the HTML tags on browsers w/o IntersectionObserver being available
				lazyAreaHtml = lazyAreaHtml.replace(
					/(?:\r\n|\r|\n|\t| )src=/g,
					' lazyload="1" src='
				);
			} else {
				if (noScriptTag.parentNode.tagName.toLowerCase() === 'picture') {
					// Temporarily prevent expensive resource loading by inserting a <source> tag pointing to a simple one (data URI)
					lazyAreaHtml =
						'<source srcset="' +
						temporaryImage +
						'" data-lazy-remove="true"></source>' +
						lazyAreaHtml;
				}

				// Temporarily replace a expensive resource load with a simple one by storing the actual source and srcset for later and point src to a temporary replacement (data URI)
				lazyAreaHtml = lazyAreaHtml
					.replace(/(?:\r\n|\r|\n|\t| )srcset=/g, ' data-lazy-srcset=')
					.replace(
						/(?:\r\n|\r|\n|\t| )src=/g,
						' src="' + temporaryImage + '" data-lazy-src='
					);
			}
		}

		return lazyAreaHtml;
	}

	/**
	 * Retrieve the elements from the 'lazy load' <noscript> tag and prepare them for display
	 * @param {Object} noScriptTag noscript HTML tag that should get initially transformed
	 */
	function prepareElement(noScriptTag) {
		// Sticking the noscript HTML code in the innerHTML of a new <div> tag to 'load' it after creating that <div>
		var lazyArea = document.createElement('div');

		lazyArea.innerHTML = getAndPrepareHTMLCode(noScriptTag);

		// Move all children out of the element
		while (lazyArea.firstChild) {
			if (
				!capabilities.loading &&
				capabilities.scrolling &&
				typeof intersectionObserver !== 'undefined' &&
				typeof document.documentElement.dataset !== 'undefined' &&
				lazyArea.firstChild.tagName &&
				(lazyArea.firstChild.tagName.toLowerCase() === 'img' ||
					lazyArea.firstChild.tagName.toLowerCase() === 'iframe')
			) {
				// Observe the item so that loading could start when it gets close to the viewport
				intersectionObserver.observe(lazyArea.firstChild);
			}

			noScriptTag.parentNode.insertBefore(lazyArea.firstChild, noScriptTag);
		}

		// Remove the empty element - not using .remove() here for IE11 compatibility
		noScriptTag.parentNode.removeChild(noScriptTag); // Preferred .removeChild over .remove here for IE
	}

	/**
	 * Get all the <noscript> tags on the page and setup the printing
	 */
	function prepareElements() {
		//
		var lazyLoadAreas = document.querySelectorAll('noscript.' + noscriptClass);

		lazyLoadAreas.forEach(prepareElement);

		// Bind for someone printing the page
		onPrinting();
	}

	// If the page has loaded already, run setup - if it hasn't, run as soon as it has.
	// Use requestAnimationFrame as this will propably cause repaints
	if (/comp|inter/.test(document.readyState)) {
		rAFWrapper(prepareElements);
	} else if ('addEventListener' in document) {
		document.addEventListener('DOMContentLoaded', function() {
			rAFWrapper(prepareElements);
		});
	} else {
		document.attachEvent('onreadystatechange', function() {
			if (document.readyState === 'complete') {
				prepareElements();
			}
		});
	}
}('loading-lazy', '256px 0px'));