Javascript: Error in FTP access

[alphaoide]

The following is a routine to download content of a file.

var req = new XMLHttpRequest();
req.open( 'GET', 'ftp://username:password@mysite.com/index.html', true );

req.onreadystatechange = function() {
  if( req.readyState == 4 ) {
    print( req.responseText );
  }
}
req.send( null );

It works fine except for the error handling. When login info is incorrect the following generic javascript window pops up.

530 Login Incorrect

When the file does not exits, it will pop up the following.

550/index.html No such file or directory

What I want to do is to suppress the generic error window and its message with my own. So how do I capture the type of error (error 530, 550, etc), and how do I prevent the generic javascript erro window from popping up?

[ober]

You'd probably be better off asking this in a web developer community.

EDIT: But AFAIK, there is no error trapping in JS.

[CornedBee]

Yes, there is. Since 1.4, I think, JavaScript supports try ... catch.

But it won't work in this situation, because that FTP handling is done internally.

I won't even talk about the absurdity of using XMLHttpRequest to get a file that is neither XML nor retrieved over HTTP.