JAVASCRIPT

set value javascript by id

document.getElementById("ID").value = "TEXT OR VARIABLE";

javascript set input value

<input type="text" id="example"> // Setup text field 
<script type="text/javascript"> 
  document.getElementById("example").setAttribute('value','My default value');
</script>

javascript set input value by id

<input type="text" id="example">
<input type="submit" value="Submit" onclick="document.getElementById('example').setAttribute('value','My default value')">