Leaflet map wont appear

Your IIFE is never invoked:

(function () {
  // some code...
}); // Function is expressed but not invoked

Should be:

(function () {
  // some code...
})(); // Make sure to add the final parenthesis pair

Just for completeness, it also works with the calling parenthesis pair just after the braces:

(function () {
  // some code...
}());