<img src="..." alt="封面">

<img class="lazyload" data-src="${list.picUrl}" alt="封面">

lazyload(document.querySelectorAll('.lazyload'))

function lazyload(images){  
    let imgs = [].slice.call(images)                   // 将dom对象变成数组  
    window.addEventListener('scroll',onscroll)         // 添加 scroll 事件监听  
    window.dispatchEvent(new Event('scroll'))     // 因为是监听scroll,所以手动触发保证第一屏的加载

    function onscroll(){        
        if(imgs.length === 0){                          // 如果没有待加载图片,移除监听      
        return window.removeEventListener('scroll',onscroll) 
    }    
    mgs = imgs.filter(img => img.classList.contains('lazyload'))   // 清洗数组    
    imgs.forEach(img => {      
        if(inViewport(img)){        
        loadImage(img)      
    }    })  }  

    function inViewport(img){                             // 判断img元素是否在视口内    
    let {top,right,bottom,left} = img.getBoundingClientRect()    
    let vpHeight = document.documentElement.clientHeight    
    let vpWidth = document.documentElement.clientWidth    
    return (      
        (top > 0 && top < vpHeight || bottom> 0 && bottom < vpHeight)       
        && (left > 0 && left < vpWidth || right > 0 && right < vpWidth)  )}  

    function loadImage(img){                              // img加载函数,加载后移除lazyload    
        let image = new Image()    
        image.src = img.dataset.src    
        image.onload = function(){      
        img.src = image.src      
        img.classList.remove('lazyload')    
        }  
    }}

<script src="https://cdn.bootcss.com/underscore.js/1.9.1/underscore-min.js"></script>

let onscroll = _.throttle(fn() , 500);

function throttle(func,wait) {      
    let previous,timer    
    return function fn(){      
    let current = Date.now()      
    let different = current - previous      
    if(!previous || different >= wait){        
        func()        
        previous = current      
    }else if(different < wait){        
        clearTimeout(timer)       
        timer = setTimeout(fn,wait - different)      
        }    
    }  
}