using select tag

by fangunxs

HTML

<form id="frm">
    <input type="text"/>
    <select>
        <option>A</option>
        <option>B</option>
    </select>
</form>

JavaScript

var frm = document.querySelector("#frm");
console.log(frm.elements);
frm.elements[1].addEventListener('change',function(){
    frm.elements[0].value = frm.elements[1][frm.elements[1].selectedIndex].value;
},false);