如何实现下载以下语句中的图片的功能？

现有一段代码：

btnSpan.innerHTML = "< img src='data:image/jpeg;base64,"+text+"' style='heigth:20px; width:20px' id='"+sysHosUUid+"_img' alt='放大' onmouseout=\"changeSmall('"+sysHosUUid+"_img');\" onmouseover=\"changeBig('"+sysHosUUid+"_img');\"

如何实现下载该字段中图片的功能，各位帮帮忙啊？

<img id='img' src='data:image/jpeg;base64,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'></img>

<script>

    function dataURLtoBlob(dataurl) {
            var arr = dataurl.split(','), mime = arr[0].match(/:(.*?);/)[1],
            bstr = atob(arr[1]), n = bstr.length, u8arr = new Uint8Array(n);
            while(n--){
              u8arr[n] = bstr.charCodeAt(n);
            }
          return new Blob([u8arr], {type:mime});
    } 

    function downloadImage(){
        var src = document.getElementById('img').src;

        var blob= dataURLtoBlob(src);   

        let url = window.URL.createObjectURL(blob);
        let link = document.createElement('a')
        link.style.display = 'none'
        link.href = url;
        link.setAttribute('download', "test.jpg")// 文件名
        document.body.appendChild(link)
        link.click()
        document.body.removeChild(link) // 下载完成移除元素
        window.URL.revokeObjectURL(url) // 释放掉blob对象
    }

    downloadImage();
</script>

<a href="data:image/jpeg;base64" download>下载</a>
有href和download就行下了，js的话可以写个a标签的click触发就下载了