js上拉加载

function loadNextPage(obj){
    this.el = obj.el;
    this.distance = this.distance ? this.distance : 0;
    this.downFN = obj.downFN;
    this.time = null;
    this.flag = true;
    this.init();
}  
loadNextPage.prototype = {
    constructor:loadNextPage,
    init:function(){        
        this.onScroll()
    },
    getScrollTop: function() {
        var scrollTop = 0;
        if(document.documentElement && document.documentElement.scrollTop) {
        scrollTop = document.documentElement.scrollTop;
        } else if(document.body) {
        scrollTop = document.body.scrollTop;
        }
        return scrollTop;
    },
    getClientHeight:function() {
        var clientHeight = 0;
        if(document.body.clientHeight && document.documentElement.clientHeight) {
        clientHeight = Math.min(document.body.clientHeight, window.innerHeight);
        } else {
        clientHeight = Math.max(document.body.clientHeight, window.innerHeight);
        }
        return clientHeight;
    },
    onScroll:function(){
        var _this = this;        
        window.onscroll = function() {  
            var elTop = document.querySelector(_this.el).getBoundingClientRect().top;          
            var elH =  document.querySelector(_this.el).getBoundingClientRect().height;
            if(_this.getScrollTop()-(_this.getScrollTop()+elTop + elH - _this.getClientHeight()) >= -1) {          
                clearTimeout(_this.time);
                _this.time=setTimeout(function(){
                    if(!_this.flag){
                        return false;
                    }
                    _this.flag = false;
                    _this.downFN()
                },300)              

            }
        }
    }
    

}

var loadNextPage = new loadNextPage({
        el:".profit_list",
        distance:10,
        downFN:function(){
             //滑动到目标元素后触发的回调函数
             //需要设置loadNextPage.flag = true才能再次回调

        }
    })