[JAVASCRIPT] Caricare Url da delle selection list

[andrespin]

Sto studiando un libro su xhtml e javascript, e sono bloccato ad un esercizio che no nriesco a completare.
Allora devo fare in modo che quando si seleziona un opzione, in una delle selection list, automaticamente si carichi l'url che è conservato nel valore value di quel'opzione.

Questo è il file esterno con gli script:

window.onload = init;

function init() {
                 var allSelect = document.getElementsByTagName("select");
                  for (var i = 0; i < allSelect.length; i++) 
                  {
                   onchange = loadLink;
                   };
                 }

function loadLink() {
                     var sIndex = ;
                     location.href = sIndex.value  ;
                     
                      }

Il libro da cui ho preso l'esercizio mi consiglia di usare, per il primo comando della funzione loadLink, la keyword this .Per il secondo comando invece di usare la variabile sIndex per riferirmi al valore dell'opzione selezionata. Vi trascrivo esattamente cosa mi chiede l'esercizio (la prima parte mi chiedeva di creare la funzione init() ma quella l'ho fatta; non riesco a capire come fare la seconda funzione):
____________________
6. Create a loadLink function.
a. Create a variable sIndex that points to the index of the selected option in the current
selection list. (hint: use the this keyword).
b. Web pages can be loaded using the command:
location.href = url
where url is the URL of the webpage. Enter this command into the function using the
value of the selected option the current selected list.(HINT: use the sIndex variable).
___________________
Non riesco a capire cosa scrivere per la variabile sIndex per farla funzionare. Scusate se sembrano un pò banali, ma ho iniziato da pochissimo a studiare javascript. Ringrazio anticipatamente chiunque mi aiuti. Ho provato ad inserire

var sIndex = this.options[this.selectedIndex];

ma non funziona. Se provo a usare la console javascript per trovare gli errori, mi dice (riferendosi a quella riga di codice):

Uncaught TypeError: Cannote read property 'undefined' of undefined



Questa è invece la parte html con le liste:
codice:

<p>Select a site from one of the drop-down lists below.</p>
<table>
   <tr><td>Executive Branch</td>
       <td><select name="executive" id="executive" class="optionLinks">
           <option value="#">Select a Web site</option>
           <option value="http://www.whitehouse.gov">The White House</option>
           <option value="http://www.usda.gov">Department of Agriculture</option>
           <option value="http://www.doc.gov">Department of Commerce</option>
           <option value="http://www.defenselink.mil">Department of Defense</option>
           <option value="http://www.ed.gov">Department of Education</option>
           <option value="http://www.energy.gov">Department of Energy</option>
           <option value="http://www.os.dhhs.gov">Department of Health and Human Services</option>
           <option value="http://www.whitehouse.gov/homeland">Department of Homeland Security</option>
           <option value="http://www.hud.gov">Department of Housing and Urban Development</option>
           <option value="http://www.doi.gov">Department of the Interior</option>
           <option value="http://www.usdoj.gov">Department of Justice</option>
           <option value="http://www.dol.gov">Department of Labor</option>
           <option value="http://www.state.gov">Department of State</option>
           <option value="http://www.ustreas.gov">Department of the Treasury</option>
           </select>
       </td>
   </tr>
   <tr><td>Legislative Branch</td>
       <td><select name="legislative" id="legislative" class="optionLinks">
           <option value="#">Select a Web site</option>
           <option value="http://www.house.gov">House Web Site</option>
           <option value="http://www.house.gov/house/MemberWWW.shtml">Representatives' Web Sites</option>
           <option value="http://clerk.house.gov/">Clerk of the House</option>
           <option value="http://www.gpoaccess.gov/cdirectory/index.html">Congressional Directory</option>
           <option value="http://www.senate.gov">Senate Web Site</option>
           <option value="http://www.senate.gov/general/contact_information/senators_cfm.cfm">Senators' Web Sites</option>
           <option value="http://www.senate.gov/artandhistory/history/common/generic/Senate_Historical_Office.htm">Senate Historic Office</option>
           <option value="http://www.cq.com">Congressional Quarterly</option>
           <option value="http://www.loc.gov/">Library of Congress</option>
           <option value="http://www.gao.gov/">Government Accountability Office</option>
           <option value="http://www.cbo.gov/">Congressional Budget Office</option>
           <option value="http://www.gpoaccess.gov/">Government Printing Office</option>
           <option value="http://thomas.loc.gov/home/legbranch/otherleg.html#govpub">Government Records and Publications</option>
           </select>
       </td>
   </tr>
   <tr><td>Judicial Branch</td>
       <td><select name="judicial" id="judicial" class="optionLinks">
           <option value="#">Select a Web site</option>
           <option value="http://www.uscourts.gov">U.S. Courts</option>
           <option value="http://www.uscourts.gov/supremecourt.html">U.S. Supreme Court</option>
           <option value="http://www.uscourts.gov/courtsofappeals.html">U.S. Courts of Appeals</option>
           <option value="http://www.uscourts.gov/districtcourts.html">U.S. District Courts</option>
           </select>
       </td>
   </tr>
   <tr><td>State Governments</td>
       <td><select name="state" id="state" class="optionLinks">
           <option value="#">Select a Web site</option>
           <option value="http://www.statelocalgov.net/index.cfm">State and Local Government on the Net</option>
           <option value="http://www.loc.gov/global/state/al-gov.html">Alabama</option>
           <option value="http://www.loc.gov/global/state/ak-gov.html">Alaska</option>
           <option value="http://www.loc.gov/global/state/az-gov.html">Arizona</option>
           <option value="http://www.loc.gov/global/state/ar-gov.html">Arkansas</option>
           <option value="http://www.loc.gov/global/state/ca-gov.html">California</option>
           <option value="http://www.loc.gov/global/state/co-gov.html">Colorado</option>
           <option value="http://www.loc.gov/global/state/ct-gov.html">Connecticut</option>
           <option value="http://www.loc.gov/global/state/de-gov.html">Delaware</option>
           <option value="http://www.loc.gov/global/state/dc-gov.html">District of Columbia</option>
           <option value="http://www.loc.gov/global/state/fl-gov.html">Florida</option>
           <option value="http://www.loc.gov/global/state/ga-gov.html">Georgia</option>
           <option value="http://www.loc.gov/global/state/hi-gov.html">Hawaii</option>
           <option value="http://www.loc.gov/global/state/id-gov.html">Idaho</option>
           <option value="http://www.loc.gov/global/state/il-gov.html">Illinois</option>
           <option value="http://www.loc.gov/global/state/in-gov.html">Indiana</option>
           <option value="http://www.loc.gov/global/state/ia-gov.html">Iowa</option>
           <option value="http://www.loc.gov/global/state/ks-gov.html">Kansas</option>
           <option value="http://www.loc.gov/global/state/ky-gov.html">Kentucky</option>
           <option value="http://www.loc.gov/global/state/la-gov.html">Louisana</option>
           <option value="http://www.loc.gov/global/state/me-gov.html">Maine</option>
           <option value="http://www.loc.gov/global/state/md-gov.html">Maryland</option>
           <option value="http://www.loc.gov/global/state/ma-gov.html">Massachusetts</option>
           <option value="http://www.loc.gov/global/state/mi-gov.html">Michigan</option>
           <option value="http://www.loc.gov/global/state/mn-gov.html">Minnesota</option>
           <option value="http://www.loc.gov/global/state/ms-gov.html">Mississippi</option>
           <option value="http://www.loc.gov/global/state/mo-gov.html">Missouri</option>
           <option value="http://www.loc.gov/global/state/mn-gov.html">Montana</option>
           <option value="http://www.loc.gov/global/state/ne-gov.html">Nebraska</option>
           <option value="http://www.loc.gov/global/state/nv-gov.html">Nevada</option>
           <option value="http://www.loc.gov/global/state/nh-gov.html">New Hampshire</option>
           <option value="http://www.loc.gov/global/state/nj-gov.html">New Jersey</option>
           <option value="http://www.loc.gov/global/state/nm-gov.html">New Mexico</option>
           <option value="http://www.loc.gov/global/state/ny-gov.html">New York</option>
           <option value="http://www.loc.gov/global/state/nc-gov.html">North Carolina</option>
           <option value="http://www.loc.gov/global/state/nd-gov.html">North Dakota</option>
           <option value="http://www.loc.gov/global/state/oh-gov.html">Ohio</option>
           <option value="http://www.loc.gov/global/state/ok-gov.html">Oklahoma</option>
           <option value="http://www.loc.gov/global/state/or-gov.html">Oregon</option>
           <option value="http://www.loc.gov/global/state/pa-gov.html">Pennsylvania</option>
           <option value="http://www.loc.gov/global/state/ri-gov.html">Rhode Island</option>
           <option value="http://www.loc.gov/global/state/sc-gov.html">South Carolina</option>
           <option value="http://www.loc.gov/global/state/sd-gov.html">South Dakota</option>
           <option value="http://www.loc.gov/global/state/tn-gov.html">Tennessee</option>
           <option value="http://www.loc.gov/global/state/tx-gov.html">Texas</option>
           <option value="http://www.loc.gov/global/state/ut-gov.html">Utah</option>
           <option value="http://www.loc.gov/global/state/vt-gov.html">Vermont</option>
           <option value="http://www.loc.gov/global/state/va-gov.html">Virginia</option>
           <option value="http://www.loc.gov/global/state/wa-gov.html">Washington</option>
           <option value="http://www.loc.gov/global/state/wv-gov.html">West Virginia</option>
           <option value="http://www.loc.gov/global/state/wi-gov.html">Wisconsin</option>
           <option value="http://www.loc.gov/global/state/wy-gov.html">Wyoming</option>
           </select>
       </td>
   </tr>
</table>