In the documentation to IndexedDB I find examples like this

// Let us open version 4 of our database
var DBOpenRequest = window.indexedDB.open("toDoList", 4);

// these two event handlers act on the database being opened
// successfully, or not
DBOpenRequest.onerror = function(event) {
  note.innerHTML += '<li>Error loading database.</li>';
};

DBOpenRequest.onsuccess = function(event) {
  note.innerHTML += '<li>Database initialized.</li>';

  // store the result of opening the database in the db
  // variable. This is used a lot later on, for opening
  // transactions and suchlike.
  db = DBOpenRequest.result;
};

As far as I understand, this first creates a request object and then defines two callbacks. But it never calls any code to execute the request. Still it seems to execute it sometime.

I am especially interested in the details, as it seems to be complicated to run different transactions after each other as transactions seem to be asynchronous.