Return value from WebMethod to javascript getting undefined error

Return value from WebMethod to javascript getting undefined error?

here iam using webmethod call in js file like

GetFields(fieldName, _ID, myEditor.onGetFieldsCompleted, myEditor.onGetFieldsFailed);

webservice.cs file
[WebMethod]
public string[] GetFields(string mergeFieldName, Int32 PartnershipID)
{
string[] stringArray = (string[])fields.ToArray(typeof(string));
return stringArray ;
}

here from webservice we are returning string array to javascript ......at tht time iam getting reult in javascript as undefined error ......pls see below js file code ........

onGetFieldsCompleted = function (result)----------here in result iam getting undefined.....actualy i have to get string array whcih i returned from webmethod in result but iam getting undefined in result...............
{
if (result != null) {
if(result.length > 0) {
}
else {

}
}

pls help me out.......