How to use a callback in Ajax

How to use a callback in Ajax

Been struggling to understand the docs for this for 24 hours. Unable to resolve it. Posted elsewhere with no luck. In fact the code I got provided caused the alert to loop continuously (maybe because the function is called when the page loads?).

I am trying to find out if a text file (note) exists on the server. I want to return "yes" if it does, "no" if it does not. I am able to get this into an alert successfully (for each of the 7 occurrences) but I want to return it back into the originating html doc so I can use it in future code. I am reading about callbacks, but not sure how to implement it.

I tried adding some callback code to the success function, that I saw in an example elsewhere but am unsure how to edit my function call:

tmpNoteFileSun is a text string that matches the format of the text files stored on the server.

The function call is loaded from: $(document).ready(function().

The function call (there are 7 of these in separate places, 1 for each day of the week):
  1. CheckNoteExist(tmpNoteFileSun);
  2. var DoesTheNoteExist = ""; //code needs to go here that returns noteexists (as in the alert below).

The Ajax Function:

  1. function CheckNoteExist(ThisNoteName, callback) {

  3. var NoteFileName = ThisNoteName;

  5.     // Ajax to call an external php file, pass the notes filename to it and check if the file
  6.     // exists. If it does, change noteexists variable to Yes", else it is "no".
  7.     $.ajax({
  8.         url: 'ajaxfile_note_exists.php',
  9.         type: 'GET',
  10.         data: {NoteFileName: NoteFileName},
  11.         success: function(noteexists) {
  12.           alert("Does the note exist: " + noteexists);
  13.           callback && callback(noteexists);
  14.         }
  15.     });

  17. }

The external PHP file:

  1. $filename = "upload/" . $_GET['NoteFileName'];

  3. if (file_exists($filename)) {
  4.   $noteexists = "yes";
  5. }
  6. else {
  7.   $noteexists = "no";
  8. }

  10. echo $noteexists;


This is the code I got elsewhere that created the alert loops:

  1. CheckNoteExist(ThisNoteName, val => console.log("Does Not Exist " + val));


Can anyone help me to get this to work please?