[javascript] Save file Javascript with file name

I'm working on a text editor in pure Javascript. I'd like it so that when the user clicks the 'Save' button, the editor downloads the file. I've already got this partly working:

uriContent = "data:application/octet-stream," + encodeURIComponent(codeMirror.getValue());
newWindow=window.open(uriContent, 'filename.txt');

The file downloads, but the problem is that the file is named 'download'.

Question: How could I change the name of the file to be anything I want, e.g filename.txt?

Use the filename property like this:

uriContent = "data:application/octet-stream;filename=filename.txt," + 
              encodeURIComponent(codeMirror.getValue());
newWindow=window.open(uriContent, 'filename.txt');

EDIT:

Apparently, there is no reliable way to do this. See: Is there any way to specify a suggested filename when using data: URI?

function saveAs(uri, filename) {
    var link = document.createElement('a');
    if (typeof link.download === 'string') {
        document.body.appendChild(link); // Firefox requires the link to be in the body
        link.download = filename;
        link.href = uri;
        link.click();
        document.body.removeChild(link); // remove the link when done
    } else {
        location.replace(uri);
    }
}