Download a client-side generated string of text.

downloadString.js

function downloadString(str){
    // get a filename
    var filename = (prompt('Filename'));

    if ('Blob' in window) {
        //create file blob
        var blob = new Blob([str], { 'type': 'text/plain;charset=utf-8' });

        if (window.navigator.msSaveBlob) {
            //works in IE 10, 11
            window.navigator.msSaveBlob(blob, filename);
        }
        else {
            //works in Chrome, Firefox
            var a = document.createElement('a');
            a.href = window.URL.createObjectURL(blob);
            a.download = filename;

            //Firefox requires the anchor to be inserted in the document
            a.style.display = 'none';
            document.body.appendChild(a);

            a.click();

            document.body.removeChild(a);
        }
    }
    else {
        //works in IE 8, 9
        if (document.execCommand) {
            var oWin = window.open('about:blank', '_blank');
            oWin.document.write(str);
            oWin.document.close();
            var success = oWin.document.execCommand('SaveAs', true, filename); //filename doesn't seem to work
            oWin.close();
            //TODO this happens when it is not possible AND when the user presses cancel
            //if (!success) alert('Saving was not successful.');
        }
        //else alert('Saving is not supported in your browser.');
    }
}