[javascript] Change value of input onchange?

I am trying to create a simple JavaScript function. When someone inserts a number in an input field, the value of another field should change to that value. Here is what I have at the moment:

function updateInput(ish) {  
    fieldname.value = ish;  
}  
<input type="text" name="fieldname" id="fieldname" />  
<input type="text" name="thingy" onchange="updateInput(value)" /> 

Somehow this does not work, can someone help me out?

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');