/**
 * 触发单击事件
 * @param elem 需要触发事件的DOM对象
 */
function fireClickEvent(elem)
{
 var event;
 if(window.MouseEvent) event = new MouseEvent('click');
 else
 {
  event = document.createEvent('MouseEvents');
  event.initMouseEvent('click', true, false, window, 0, 0, 0, 0, 0, false, false, false, false, 0, null);
 }
 elem.dispatchEvent(event);
}

<a href="data:text/txt;charset=utf-8,测试下载纯文本" rel="external nofollow" rel="external nofollow" rel="external nofollow" download="测试.txt" >下载1</a>
<a href="data:text/txt;charset=utf-8,测试下载纯文本" rel="external nofollow" rel="external nofollow" rel="external nofollow" >下载2</a>

openDownloadDialog(url, saveName)
openDownloadDialog(blob, saveName)

/**
 * 通用的打开下载对话框方法，没有测试过具体兼容性
 * @param url 下载地址，也可以是一个blob对象，必选
 * @param saveName 保存文件名，可选
 */
function openDownloadDialog(url, saveName)
{
 if(typeof url == 'object' && url instanceof Blob)
 {
  url = URL.createObjectURL(url); // 创建blob地址
 }
 var aLink = document.createElement('a');
 aLink.href = url;
 aLink.download = saveName || ''; // HTML5新增的属性，指定保存文件名，可以不要后缀，注意，file:///模式下不会生效
 var event;
 if(window.MouseEvent) event = new MouseEvent('click');
 else
 {
  event = document.createEvent('MouseEvents');
  event.initMouseEvent('click', true, false, window, 0, 0, 0, 0, 0, false, false, false, false, 0, null);
 }
 aLink.dispatchEvent(event);
}

姓名,期中成绩,期末成绩
张三,58,95
李四,98,74
王二,47,38
刘能,15,100
黄五,87,68

<a download="测试.txt" href="data:text/txt;charset=utf-8,测试下载纯文本" rel="external nofollow" rel="external nofollow" rel="external nofollow" >下载</a>

<button onclick="test()">下载CSV</button>
<script>
function test()
{
 var csv = '姓名,期中成绩,期末成绩\n张三,58,95\n李四,98,74';
 var a = document.createElement('a');
 a.href = 'data:text/txt;charset=utf-8,'+encodeURIComponent(csv);
 a.download = '测试.csv';
 a.click(); // 这里偷个懒，直接用click模拟
}
</script>

<button onclick="test()">下载CSV</button>
<script>
function test()
{
 var csv = '姓名,期中成绩,期末成绩\n张三,58,95\n李四,98,74';
 var a = document.createElement('a');
 a.href = 'data:text/txt;charset=utf-8,\ufeff'+encodeURIComponent(csv);
 a.download = '测试.csv';
 a.click(); // 这里偷个懒，直接用click模拟
}
</script>

var csv = '姓名,期中成绩,期末成绩\n张三,58,95\n李四,98,74';
var blob = new Blob(['\ufeff' + data], {type: 'text/csv,charset=UTF-8'});
openDownloadDialog(blob, '测试.csv')；

/**
 * 保存CSV文件
 * @params csv csv文件内容
 * @params saveName 保存的文件名
 */
function saveCSV(csv, saveName)
{
 var a = document.createElement('a');
 a.href = 'data:text/csv;charset=utf-8,\ufeff' + encodeURIComponent(csv);
 a.download = saveName;
 a.click();
}

/**
 * 保存CSV文件
 * @params csv csv文件内容
 * @params saveName 保存的文件名
 */
function saveCSV(csv, saveName)
{
 var blob = new Blob(['\ufeff' + csv], {type: 'text/csv,charset=UTF-8'});
 openDownloadDialog(blob, saveName)；
}

var csv = '你好，我是小茗同学！\n测试换行！';
var blob = new Blob([data], {type: 'text/txt,charset=UTF-8'});
openDownloadDialog(blob, '测试.csv')；

/**
 * 将某个canvas保存为图片
 * @param canvasObj canvas对象
 * @param saveName 保存的名称
 * @param type 保存的图片格式，如 image/png
 * @param quality 图片质量，可选0-1
 */
function saveImage(canvasObj, saveName, type, quality)
{
 if(!canvasObj) return;
 type = type || 'image/png';
 quality = quality || 0.92;
 var url = canvasObj.toDataURL(type, quality).replace(/image\/.*?;/, 'image/octet-stream;');
 openDownloadDialog(url, saveName);
}