Mandatory field population based on another field value

All,

I've attempted to scour the posts here to see if there's anything out there, but I'm not finding what I need.

I'm attempting to have a field be mandatory if another field has a certain value. So if the Status on a client record has one of two values, and a custom field is null or empty, prompt the user to fill in the custom field.

Here's the code I have:

function FieldChanged(type, name)      var status = nlapiGetFieldText('entitystatus')  var reason = nlapiGetFieldText('custentity_reason_closed_field')    {   if  ((status == "Prospect-Closed") || ("customer-Not Renewed") && (reason == null) || (""));     {    alert ("please enter a Reason Code")   }    }