Javascript if else condition for fields depending on values selected

I have a problem with a condition statement with 4 fields involved

1. Issuing country select field [ Select field ]
2. Currency of the amount to be paid [ Select Field ]

3. Conversion of the amount entered [ Input Field ]

   //if "AUS" is selected auto-select AUD as currency and run the second function to the amt_input field
   
   
   
   <select name="issuing_country" id="issuing_country">
       <option value=""> -- Please Select --</option>
       <option id="ARG" value="ARG">Argentina</option>
       <option id="USA" value="ARM">Armenia</option>
       <option id="ABW" value="ABW">Aruba</option>
       <option id="AUS" value="AUS">Australia</option>
    <select> 
   
   
   
   //Select a currency:
   
   <tr class="shade">
      <td align="left">Currency: &nbsp;</td>
      <td>
         <select name="vpc_Currency">
            <option value="USD">USD</option>
            <option value ="AUD" selected>AUD</option>
         </select>
      </td>
   </tr>
   
   

   //where to enter the amount//

    <input type="text" name="amt" value="">
   
   

   //field that will reflect the computation//

     <input id="Total_Amount" name="vpc_Amount" value=""/>
   
   
   //Javascript//
   
   <script type="text/javascript">
   
      function ().call(){
   
            var amt;
               function.do{
   
   

   //if USD is selected to be converted to AUD in the amt field//

                   if (vpc_Currency = "USD")&&(AUS)
                   var amount = (usd * 1.36) * 1.023;
                   return amount;
   
   

   //if AUD currency and issuing bank is AUS//

            else if (vpc_Currency = "AUD")&&(loc='AUS')
                   var amount = [(aud * 1.023) * 1.1]
                   return amount; 
   
   

   //if different country and bank automatic select USD as currency//

                   else (aud * 1.023);
                   return amount;
               }
        }