JS scope question

loadJSON.js

      // Taken from https://stackoverflow.com/questions/9838812/how-can-i-open-a-json-file-in-javascript-without-jquery
      function loadJSON(path, success, error) {
        var xhr = new XMLHttpRequest();
        xhr.onreadystatechange = function()
        {
          if (xhr.readyState === XMLHttpRequest.DONE) {
            if (xhr.status === 200) {
              if (success) {
                success(JSON.parse(xhr.responseText));
              }
            } else {
              if (error) {
                error(xhr);
              }
            }
          }
        };
        xhr.open('GET', path, true);
        xhr.send();
      }

usage.js

(function() {
  tinymce.create('tinymce.plugins.InsertExercise', {
    InsertExercise: function(ed, url) {
      ...
      
      function showDialog() {
      ...

        loadJSON(exercises_url,
          function(data) {
            /* I need to access "ed" here! Possible somehow? */
          },
          function(xhr) {console.error(xhr);}
        );
      
      ...