how to make Correctly a javascript if exist (folder) command?

Question

I have been trying for some time to design a script.

In this script I will look for commands with an if query whether a folder exists.

I do this by using a varia

Answer 1

var folderExists = function(callback) {
  var eld = "default";
  $.get( "backgrounds/" + mapname )
  .done(function() { 
    eld = mapname;
    callback(eld);
  }).fail(function() { 
    callback(eld);
  });    
}

folderExists(function(folderName) {
  if (folderName === "default") {
    alert('Folder doesnt exist apparently...')
  }
}

Not that I would ever check the existence of a folder like this, but I might be missing some things.