lazy loading image code

When using lazyload.js and swiper.js together, it is found that the loaded image does not load after adding it to one screen. I can't find the reason. I can use the following code. It is available for personal testing, and the mobile terminal and WeChat are normal.

var scrollElement = document.querySelector('.page'),
            viewH = document.documentElement.clientHeight;

        function lazyload() {
            var nodes = document.querySelectorAll('img[data-src]');
            Array.prototype.forEach.call(nodes, function (item, index) {
                var rect;
                if (item.dataset.src === '') return;
                rect = item.getBoundingClientRect();
                if (rect.bottom >= 0 && rect.top < viewH) {
                    (function (item) {
                        var img = new Image();
                        img.onload = function () {
                            item.src = img.src;
                        }
                        img.src = item.dataset.src;
                        item.dataset.src = '';
                    })(item)
                }
            })
        }

        function throttle(fun, delay, time) {
            var timeout,
                startTime = new Date();
            return function () {
                var context = this,
                    args = arguments,
                    curTime = new Date();
                clearTimeout(timeout);
                if (curTime - startTime >= time) {
                    fun.apply(context, args);
                    startTime = curTime;
                } else {
                    timeout = setTimeout(fun, delay);
                }
            };
        };

        $(function () {
            //动态加载元素时写在这里面更保险一些
            lazyload();
            //scrollElement为图片外面的父元素
            scrollElement.addEventListener('scroll', throttle(lazyload, 500, 1000));
        })