SOURCE

function fake_click(obj) {
    var ev = document.createEvent("MouseEvents");
    ev.initMouseEvent(
        "click", true, false, window, 0, 0, 0, 0, 0, false, false, false, false, 0, null
    );
    obj.dispatchEvent(ev);
}


function download(name, data) {
    var urlObject = window.URL || window.webkitURL || window;

    var downloadData = new Blob([data]);

    var save_link = document.createElementNS("http://www.w3.org/1999/xhtml", "a")
    save_link.href = urlObject.createObjectURL(downloadData);
    save_link.download = name;
    fake_click(save_link);
}
//调用方法


var converter = new showdown.Converter(),
    text      = '# hello, markdown!',
    html      = converter.makeHtml(text);
    console.log(html);

download("save.html",html);