Using server-side JavaScript in a YQL open table you can shift JavaScript logic to the server side. In this example I wanted to write a script that loads an HTML document and counts the amount of times a certain tag is used. With pure JavaScript this can't be done because of cross-domain loading issues. This is why I wrote the following XML document and put it on my server:

<?xml version="1.0" encoding="UTF-8"?>
<table xmlns="http://query.yahooapis.com/v1/schema/table.xsd">
  <meta>
    <sampleQuery>select * from count where url="http://www.korea-dpr.com/" and tag="strong"</sampleQuery>
    <author>Christian Heilmann</author>
    <documentationURL>http://isithackday.com/hackday-toolbox/yql-open-table-with-js</documentationURL>  
    <description>Counts to amount of times a certain tag is in an HTML document.</description>
  </meta>
  <bindings>
    <select itemPath="" produces="XML">
      <inputs>
        <key id='url' type='xs:string' paramType='variable' required="true" />
        <key id='tag' type='xs:string' paramType='variable' required="true" />
      </inputs>
      <execute><![CDATA[
        var data = y.rest(url).get().response;
        var match = new RegExp('<'+tag+'[>|\s]');
        var count = data.split(match);
        var number = parseInt(count.length-1);
        response.object = <counted>
                            <url>{url}</url>
                            <tag>{tag}</tag>
                            <count>{number}</count>
                          </counted>;
      ]]></execute>
    </select>
  </bindings>
</table>

The y.rest() command loads external data for me on the server and then allows me to manipulate it in JavaScript. In this case, I get the content and split it at the tag I want to count. Then I return an object which is all the data as plain XML (as YQL supports E4X).

In YQL, you can apply this table with the use command:

use "http://isithackday.com/hackday-toolbox/yql-open-table-with-js/count-tags.xml" as count;
select * from count where url="http://www.korea-dpr.com/" and tag="strong"

The results would be:

<?xml version="1.0" encoding="UTF-8"?>
<query xmlns:yahoo="http://www.yahooapis.com/v1/base.rng" yahoo:count="1" yahoo:created="2010-07-29T11:58:03Z" yahoo:lang="en-US">
  <diagnostics>
    <publiclyCallable>true</publiclyCallable>
    <url execution-time="3" proxy="DEFAULT"><![CDATA[http://isithackday.com/hackday-toolbox/yql-open-table-with-js/count-tags.xml]]></url>
    <url execution-time="244" proxy="DEFAULT"><![CDATA[http://www.korea-dpr.com/]]></url>
    <javascript execution-time="306" instructions-used="3095" table-name="count"></javascript>
    <user-time>313</user-time>
    <service-time>247</service-time>
    <build-version>8771</build-version>
  </diagnostics>
  <results>
    <counted>
      <url>http://www.korea-dpr.com/</url>
      <tag>strong</tag>
      <count>904</count>
    </counted>
  </results>
</query>

See the source of this page to check how to use this in YUI3. Find out more about server-side JavaScript in YQL here.