client script to validate custom field

client script to validate custom field

I'm new to Zoho CRM. I'm trying to solve a problem. Here is my pseudo code

create a new deal
serial_number = user input
check serial number (user input) against serial number in all records in product module
if serial number (user input) not found in product module then
  generate error message "product does not exist and needs to be created"
else
 assign field values from products to field values in deals

basically, in the custom field 'serial number' i want to validate user input against serial number for products (another module). 

(I don't expect the script to create the new product record, I can get the user to do that. I just need to know if the serial number currently exists in a record in the products module and if not then display message)

what I don't know:
  1. how to code this
  2. if client script (client based) can work to retrieve data on the server for comparison (do I need a function for that)
can anyone point me in the direction of similar code? I have a difficult time coding from scratch but I can walk through and make changes to get it to work! ;-)

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