js selezionare il valore di modifica
document.getElementById('my-select').addEventListener('change', function() {
console.log('You selected: ', this.value);
});
onchange su seleziona tag
<select id="comboA" onchange="getComboA(this)">
<option value="">Select combo</option>
<option value="Value1">Text1</option>
<option value="Value2">Text2</option>
<option value="Value3">Text3</option>
</select>
modifica del valore di input javascript
function updateInput(ish){
document.getElementById("fieldname").value = ish;
}