Change value of input onchange?

You can't access your fieldname as a global variable. Use document.getElementById:

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

and

onchange="updateInput(this.value)"

<input type="text" name="fieldname" id="fieldtobechanged" />  
<input type="text" name="thingy"  id="inputfield" />

I have used following code and it works instantly without any delay.

var timeoutID = null;
    
function findMember(str) {
    document.getElementById("fieldname").innerHTML = str;
}

$('#inputfield').keyup(function(e){
    clearTimeout(timeoutID);
    timeoutID = setTimeout(findMember.bind(undefined, e.target.value), 500);
});

for jQuery we can use below:

by input name:

$('input[name="textboxname"]').val('some value');

by input class:

$('input[type=text].textboxclass').val('some value');

by input id:

$('#textboxid').val('some value');