メモ
html
<textarea id="txtarea"></textarea>
<a href="" id="DLlink" download="download.txt">ダウンロード</a>
JavaScript
document.querySelector('#DLlink').onclick = function() {
var text = document.querySelector('#txtarea').value;
this.href = 'data:text/plain;charset=utf-8,'
+ encodeURIComponent(text);
};