Now let's say that the connection to the database was down when the user first entered the application. The function in Listing 1 is used to get a list of all the departments in the company.

If the database server is down the cfcatch will catch a database error and throw a new error message that will tell the user that the database server is down, it's up to the page that calls this function to trap the error and set the session.error and session.errorMessage variables:

<cfscript>
    try
    {
      Department = CreateObject("Component","CFDJ.Components.Department");
      DepartmentArray = Department.GetAllDepartments(#session.dsn#);
    }
    catch(Any ex)
    {
      session.error = true;
      session.errorMessage = ex.Message;
    }
</cfscript>

In this case the employee update page would look like Figure 4 when the user enters the application.

Next let's look at using JavaScript to do some simple client-side error handling. The first step is to check for invalid data. I'll assume that you've had some experience doing simple JavaScript error checking and skip right to displaying the error. For our example, we're going to have the JavaScript check to make sure that the user has selected the name of a user to edit from the user drop-down. The first step would be to make sure that the selected index of a drop-down is not zero. Typically the next step would be to display some sort of message to the user with the Alert() function. In this case we'll simply call the ShowError() function that we wrote above:

if(departmentSelect.selectedIndex != 0)
{
      //do something
}
else
{
      ShowMessage('Please select an employee to update',true);
}

Conclusion
Adding a messaging system to an AJAX application isn't a very complicated, but it can be an extremely useful technique for keeping your end users informed. This application can be viewed online at www.innovacreative.com/cfdj/ajax. The source code can be downloaded from www.innovacreative.com/cfdj/ajax/ajax.zip.