方法一：
 <a href="https://fuss10.elemecdn.com/e/5d/4a731a90594a4af544c0c25941171jpeg.jpeg" download></a>
 存在跨域问题，通过download 属性下载文件，必须为同一域名下，否则无效
方法二：
downloadImg (url,filename) {
 let canvas = document.createElement('canvas')
 let context = canvas.getContext('2d')

 let aLink = document.createElement('a')
     aLink.download = filename
     aLink.style.display = 'none'
 let img = new Image;
    //设置跨域
    img.setAttribute('crossOrigin', 'anonymous')
    img.src=url
    img.onload = function(){
       canvas.width = img.width
       canvas.height = img.height
       context.drawImage(img,0,0);
       aLink.href = canvas.toDataURL('image/jpeg')
       // 触发点击
       document.body.appendChild(aLink)
       aLink.click();
       // 然后移除
       document.body.removeChild(aLink)
    }
}

其他：https://www.cnblogs.com/haojf/p/9407183.html