Ajax Request in drupal Module by javascript

Change value of dependent fields in drupal by Ajax request

We have to change a field value when changing other field. Drupal Ajax menu is right method, but I found other way easier.

hook_menu :- Create a url with hook_menu.

page_callback :- In page callback we have to process all things and return data (in json format) that will need in javascript.

drupal_add_js :- Now use $.get method in js and hit that url and parse json ($.parseJSON) data from response and perform other actions.

/*======== Code Example==========*/

/**
 * Implements hook_menu().
 */
function test_menu() {
  return array(
    'test' => array(
      'title' => 'Test',
      'page callback' => 'test_page',
      'type' => MENU_CALLBACK,
      'access callback' => TRUE,
    ), 
  ); 
} 

/**
 * callback function.
 */
function test_page() {
  $data = array('greeting' => 'hello', 'name' => 'json');
  $json_data = json_encode($data);
  print_r($json_data); die();
}

If you want to receive only data required, nothing else in javascript-end then use die(); otherwise return $json_data;  
drupal_add_js('
  $("#selectfield").change(function() {
    $.get(Drupal.settings.basePath+"test/", function(response) {
      var resp = $.parseJSON(response);
      console.log(resp);
      $("textfield").val(resp.greeting+" "+resp.name).change();
    });
  });', 'inline');

That's it.