<a href="http://website.com/example_1.txt" download="saved_as_filename.txt" id="downloader">
    static download link</a>

var theURL = 'http://foo.com/example_2.txt';
$('#downloader').click(function() {
    $(this).attr('href',theURL);
});

$('#downloader').css('display','none');
function initiateDownload(someURL) {
     theURL = someURL;
     var event = document.createEvent("MouseEvent");
     event.initMouseEvent("click", true, true, window, 0, 0, 0, 80, 20, false, false, false, false, 0, null);
     // dispatch event won't work on a jQuery object, so use getElementById
     var el = document.getElementById('downloader');
     el.dispatchEvent(event);
}
initiateDownload('example_3.txt');