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AJAX helps you create better, faster and more user-friendly web applications with its Javascript and HTTP base. It allows the page content to be updated without reloading a whole page.

In Drupal, AJAX functionality is provided through functions in the Javasript file drupal.js, and some AJAX functions are supported by Drupal core.

I am going to explain the autocomplete which is one of the AJAX functions implemented in Drupal. When users type into a text filed, new data from the server is dynamically loaded and displayed as a list of matching options. Exchanging data beteween client and server, and updating the content are handled by the autocomplete.js. The textfield type form input has the #autocomplete_path attribute which is the path that the AJAX autocomplete script uses as the source for autocompletion. You can easily implement the AJAX autocomplete to your form using the #autocomplete_path attribute.

Implementing the autocomplete contains two parts:1) a form input textfield and 2) a handler function to parse the request and return a response. There are build-in autocomplete handler functions such as user_autocomplete() and taxonomy_autocomplete().

Here is a simple example of how to implement the autocomplete uisng the user_autocomplete() function.

function example_form(){  
  $form['item'] = array(   
    '#type' => 'textfield',   
    '#autocomplete_path' => 'user/autocomplete'  
    ); 
  }

This user/autocomplete path calls the user_autocomplete() function and loads matching user names.

Also, you can create a custom autocomplete handler to meet a need for your site. Let’s say you have a text field which asks users to select a node item on the site.

function example_form(){  
  $form['item'] = array(   
    '#type' => 'textfield',   
    '#autocomplete_path' => 'example/autocomplete' 
    ); 
  }

function example_menu($may_cache) {  
  $item[] = array(   
    'path' => 'example/autocomplete',   
    'callback' => 'example_autocomplete',   
    'access' => user_access('access example autocomplete')  
    ); 
  }

function example_autocomplete($string){
  $items = array();  
  $result = db_query("SELECT nid, title FROM {node} WHERE status = 1 AND title LIKE ''%s%%', $string");
  
  while($obj = db_fetch_object($result)) {
    $items[$obj->nid] = check_plain($obj->title);
  }
  print drupal_to_js($items); 
  exit();
}

User input is passed from the parameter and the array of matching items is printed as the response. Note that you need to escape the node titles using check_plain() because the values are HTML.

Also note that you need to use your autocomplete handler callback has proper access permission.It might be exposing some sensitive information on the site.