Script
function saveAsFile(str, filename) {
var hiddenElement = document.createElement('a');
hiddenElement.href = 'data:attachment/text,' + encodeURI(str);
hiddenElement.target = '_blank';
hiddenElement.download = filename;
hiddenElement.click();
}
var strdata = "Hello, world!";
saveAsFile(strdata, "output.txt");
Demo
click: download file