图片懒加载

            check(document.querySelectorAll('img.J_lazy'))
            function isIn(el) {
                var bound = el.getBoundingClientRect();
                var clientHeight = window.innerHeight;
                return bound.top <= clientHeight;
            }
            //检查图片是否在可视区内，如果不在，则加载
            function check(item) {
                Array.from(item).forEach(function(el){
                    console.log(777)
                    if(isIn(el)){
                        loadImg(el);
                    }
                })
            }
            function loadImg(el) {
                if(el.getAttribute('class').indexOf('J_lazy') > -1){
                    var source = el.dataset.original;
                    el.src = source;
                    el.classList.remove("J_lazy");
                }
            }
            function isIn(el) {
                var bound = el.getBoundingClientRect();
                var clientHeight = window.innerHeight;
                return bound.top <= clientHeight;
            }