https://www.thebct.org.uk/js/plugins/lazyload/lazyload.js?v=4

1/*!
2 * Lazy Load - JavaScript plugin for lazy loading images
3 *
4 * Copyright (c) 2007-2019 Mika Tuupola
5 *
6 * Licensed under the MIT license:
7 *   http://www.opensource.org/licenses/mit-license.php
8 *
9 * Project home:
10 *   https://appelsiini.net/projects/lazyload
11 *
12 * Version: 2.0.0-rc.2
13 *
14 */
15
16(function (root, factory) {
17    if (typeof exports === "object") {
18        module.exports = factory(root);
19    } else if (typeof define === "function" && define.amd) {
20        define([], factory);
21    } else {
22        root.LazyLoad = factory(root);
23    }
24}) (typeof global !== "undefined" ? global : this.window || this.global, function (root) {
25
26    "use strict";
27
28    if (typeof define === "function" && define.amd){
29        root = window;
30    }
31
32    const defaults = {
33        src: "data-src",
34        srcset: "data-srcset",
35        selector: ".lazyload",
36        root: null,
37        rootMargin: "0px",
38        threshold: 0
39    };
40
41    /**
42    * Merge two or more objects. Returns a new object.
43    * @private
44    * @param {Boolean}  deep     If true, do a deep (or recursive) merge [optional]
45    * @param {Object}   objects  The objects to merge together
46    * @returns {Object}          Merged values of defaults and options
47    */
48    const extend = function ()  {
49
50        let extended = {};
51        let deep = false;
52        let i = 0;
53        let length = arguments.length;
54
55        /* Check if a deep merge */
56        if (Object.prototype.toString.call(arguments[0]) === "[object Boolean]") {
57            deep = arguments[0];
58            i++;
59        }
60
61        /* Merge the object into the extended object */
62        let merge = function (obj) {
63            for (let prop in obj) {
64                if (Object.prototype.hasOwnProperty.call(obj, prop)) {
65                    /* If deep merge and property is an object, merge properties */
66                    if (deep && Object.prototype.toString.call(obj[prop]) === "[object Object]") {
67                        extended[prop] = extend(true, extended[prop], obj[prop]);
68                    } else {
69                        extended[prop] = obj[prop];
70                    }
71                }
72            }
73        };
74
75        /* Loop through each object and conduct a merge */
76        for (; i < length; i++) {
77            let obj = arguments[i];
78            merge(obj);
79        }
80
81        return extended;
82    };
83
84    function LazyLoad(images, options) {
85        this.settings = extend(defaults, options || {});
86        this.images = images || document.querySelectorAll(this.settings.selector);
87        this.observer = null;
88        this.init();
89    }
90
91    LazyLoad.prototype = {
92        init: function() {
93
94            /* Without observers load everything and bail out early. */
95            if (!root.IntersectionObserver) {
96                this.loadImages();
97                return;
98            }
99
100            let self = this;
101            let observerConfig = {
102                root: this.settings.root,
103                rootMargin: this.settings.rootMargin,
104                threshold: [this.settings.threshold]
105            };
106
107            this.observer = new IntersectionObserver(function(entries) {
108                Array.prototype.forEach.call(entries, function (entry) {
109                    if (entry.isIntersecting) {
110                        self.observer.unobserve(entry.target);
111                        let src = entry.target.getAttribute(self.settings.src);
112                        let srcset = entry.target.getAttribute(self.settings.srcset);
113                        if ("img" === entry.target.tagName.toLowerCase()) {
114                            if (src) {
115                                entry.target.src = src;
116                            }
117                            if (srcset) {
118                                entry.target.srcset = srcset;
119                            }
120                        } else {
121                            entry.target.style.backgroundImage = "url(" + src + ")";
122                        }
123                    }
124                });
125            }, observerConfig);
126
127            Array.prototype.forEach.call(this.images, function (image) {
128                self.observer.observe(image);
129            });
130        },
131
132        loadAndDestroy: function () {
133            if (!this.settings) { return; }
134            this.loadImages();
135            this.destroy();
136        },
137
138        loadImages: function () {
139            if (!this.settings) { return; }
140
141            let self = this;
142            Array.prototype.forEach.call(this.images, function (image) {
143                let src = image.getAttribute(self.settings.src);
144                let srcset = image.getAttribute(self.settings.srcset);
145                if ("img" === image.tagName.toLowerCase()) {
146                    if (src) {
147                        image.src = src;
148                    }
149                    if (srcset) {
150                        image.srcset = srcset;
151                    }
152                } else {
153                    image.style.backgroundImage = "url('" + src + "')";
154                }
155            });
156        },
157
158        destroy: function () {
159            if (!this.settings) { return; }
160            this.observer.disconnect();
161            this.settings = null;
162        }
163    };
164
165    root.lazyload = function(images, options) {
166        return new LazyLoad(images, options);
167    };
168
169    if (root.jQuery) {
170        const $ = root.jQuery;
171        $.fn.lazyload = function (options) {
172            options = options || {};
173            options.attribute = options.attribute || "data-src";
174            new LazyLoad($.makeArray(this), options);
175            return this;
176        };
177    }
178
179    return LazyLoad;
180});