Controls > Ajax.Autocompleter


Ajax.Autocompleter allows for server-powered autocompleting text fields.


new Ajax.Autocompleter(id_of_text_field, id_of_div_to_populate, url, options);

Options (Inherited from Autocompleter.Base, as well as all options for Prototype’s Ajax.Request )

Option Default Value Description
paramName the ‘name’ of the element Name of the parameter for the string typed by the user on the autocompletion field
tokens [] Tokenized incremental autocompletion is enabled automatically when an autocompleter is instantiated with the ‘tokens’ option in the options parameter: See Also Autocompleter.Base
frequency 0.4 How frequently (in seconds) the input field should be polled for changes before firing an Ajax request.
minChars 1 The minimum number of characters that must be entered in the input field before an Ajax request is made.
select null The class name of the element that contains the text to be placed into the input box. By default all text will be used.
indicator null The HTML id of an element to display (using Element.show) while the Ajax request is in progress. This element will be hidden with Element.hide when the request is completed. This is useful for displaying an animated spinner during processing. See Ajaxload for some image examples.
updateElement null Hook for a custom function to replace the built-in function that adds the list item text to the input field. The custom function is called after the element has been updated (i.e. when the user has selected an entry). The function receives one parameter only: the selected item (the <li> item selected)
afterUpdateElement null Hook for a custom function that’s called following the execution of the built-in function that adds the list item text to the input field, which happens after a user has selected an entry. (The difference between updateElement and afterUpdateElement is that updateElement replaces the built-in Autocompleter function; afterUpdateElement supplements that built-in function.) The function receives two parameters, the input field specified for autocompletion, and the selected item (the <li> item selected)
callback null This function is called just before the Request is actually made, allowing you to modify the querystring that is sent to the server. The function receives the completer’s input field and the default querystring (‘value=XXX’) as parameters. You should return the querystring you want used, including the default part.
parameters null To send additional parameters to the server, add them here in the format: 'field=value&another=value'. Errata/bug: The hash format {field: 'value',another: 'value'} was noted in this wiki to work, but it does not, as the Ruby on Rails project has separately documented and fixed in their codebase.



<input type="text" id="autocomplete" name="autocomplete_parameter"/>
<div id="autocomplete_choices" class="autocomplete"></div>


new Ajax.Autocompleter("autocomplete", "autocomplete_choices", "/url/on/server", {});

HTML with indicator

<input type="text" id="autocomplete" name="autocomplete_parameter"/>
<span id="indicator1" style="display: none">
  <img src="/images/spinner.gif" alt="Working..." />
<div id="autocomplete_choices" class="autocomplete"></div>

(As with any element destined to work with Prototype’s Element.toggle/show/hide, your indicator element should use an inline style attribute with a display property, here initially set to none. If you need to assign it CSS rules, put the class attribute before the style attribute to avoid override.)

Javascript with options

new Ajax.Autocompleter("autocomplete", "autocomplete_choices", "/url/on/server", {
  paramName: "value", 
  minChars: 2, 
  indicator: 'indicator1'

The styling of the div and the returned UL are important.
Applying a visual cue that an item is selected allows the user to take advantage of the keyboard navigation of the dropdown and adding background colors, borders, positioning, etc to the div (as the demo does) allows the UI element to stand out. The CSS from the demo applied to the examples would be:

div.autocomplete {
  border:1px solid #888;
div.autocomplete ul {
div.autocomplete ul li.selected { background-color: #ffb;}
div.autocomplete ul li {

Tokenized autocompletion

Tokenized incremental autocompletion is enabled automatically when an autocompleter is instantiated with the ‘tokens’ option in the options parameter:

new Ajax.Autocompleter('id','upd', 'url', { tokens: ',' });

will incrementally autocomplete with a comma as the token.

Additionally, ’,’ in the above example can be replaced with a token array, e.g. tokens: [',', '\n'] which enables autocompletion on multiple tokens. This is most useful when one of the tokens is \n (a newline), as it allows smart autocompletion after linebreaks.

Server Return

Look through your POST environment variable for the current entry in the text-box.

The server-side will receive the typed string as a parameter with the same name as the name of the text field element of the autocompletion (its name attribute). You can override it by setting the option paramName.

The server must return an unordered list.
For instance, this list might be returned after the user typed the letter “y”

    <li>your mom</li>

If your search returns no results, it is critical that your server immediately return an empty list, rather than nothing. You will only notice this need if you use an indicator to show server activity — the indicator will never go away if your server doesn’t return an empty list.

If you wish to display additional information in the autocomplete dropdown that you don’t want inserted into the field when you choose an item, surround it in a <span> (could work with others but not tested) with the class of “informal”.

For instance, the following shows a list of companies and their addresses, but only the company name will get inserted:

    <li>ACME Inc <span class="informal"> 5012 East 5th Street</span></li>
    <li>Scriptaculous <span class="informal"> 1066 West Redlands Parkway</span></li>

Another way to get additional information is to encode an ID into each list item, and redefine the afterUpdate Element to handle that ID:

    <li id="1">your mom</li>
    <li id="2">yodel</li>
new Ajax.Autocompleter("autocomplete", "autocomplete_choices", "/url/on/server", {
  afterUpdateElement : getSelectionId

function getSelectionId(text, li) {
    alert (li.id);


When a choice is highlighted, the Autocompleter applies a class of “selected” to the li element. This allows the use of CSS to style the selected element.


A short tutorial on using Autocompleter, together with Ruby and WEBrick, prepared for the Linux Users Group, Villafranca, Verona, Italy.