You cannot do this in that manner using JS. What you CAN do though is to download it on the client, without having the server intervene by using data urls
.
Setting the name of said downloaded file is not cross browser compatible unfortunately...
HTML
<textarea id="text" placeholder="Type something here and click save"></textarea>
<br>
<a id="save" href="">Save</a>
Javascript
// This defines the data type of our data. application/octet-stream
// makes the browser download the data. Other properties can be added
// while separated by semicolons. After the coma (,) follows the data
var prefix = 'data:application/octet-stream;charset=utf-8;base64,';
$('#text').on('keyup', function(){
if($(this).val()){
// Set the URL by appending the base64 form of your text. Keep newlines in mind
$('#save').attr('href', prefix + btoa($(this).val().replace(/\n/g, '\r\n')));
// For browsers that support it, you can even set the filename of the
// generated 'download'
$('#save').attr('download', 'text.txt');
}else{
$('#save').removeAttr('href');
$('#save').removeAttr('download');
}
});