<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
</head>
<body>
<input id="txt" type="text" />
<input id="btn" type="button"/>
<script>
    var oTxt = document.getElementById('txt' );
    var oBtn = document.getElementById('btn');
    oBtn.onclick = function ()
    {
//       oTxt.value = 'defdi';
//        oTxt['value'] = 'deidi';
        oTxt.setAttribute('value','dieid');
    };
</script>
</body>
</html>