select Onchange js

document.getElementById("myInputID").addEventListener("change", function(){
	//This input has changed
   console.log('This Value is', this.value);
});

<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>

<!-- 
	You can use any of the following:
	onkeydown, onkeyup or onchange
 -->

<input type="text" onkeydown="keydownFunction()" 
onkeyup="keyupFunction()" onchange="changeFunction()">

<input type="text" id="username" onchange="usernameChanged">

function updateInput(ish){
    document.getElementById("fieldname").value = ish;
}

 <select onchange="myFunction()">