Populate lookup field based on another lookup field on the form.

I have two lookup on the form lets Say A  and B.

I need to populate B based on the value in A as A has a main form where this lookup B is populated with the value and that value should populate in my form where both this field are present

I cannot use mapping for it as B and A is 1:N related and user is not allowed to fill B first so that we can populate A. but vice versa is needed.

I hope I have cleared my requirements

Can i get any JavaScript sample code . I am pretty new to it so needed help

Thanks in advance.