Have I to reload every time to default a field ?

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Hi,

I am working on a Request a New Position Page and have a logic when user choose job ,if it has validgrade it retrieves grade code and default another field with gradecode, if not then user select

a valid grade and retrieve it's grade code  default another field with gradecode, the issue I am facing is when user choose job with valid grades then choose another job without validgrades

the default field become null and I have to reload the page again every time , how to handle this ?

your support is highly appreciated.