<body>
<input type="text" id="a1"><br>
<input type="text" id="a2"><br>
<input type="button" value="length" onClick="lengthvalue()">
<script>
function lengthvalue()
{
a=document.getElementById("a1").value;
b=a.length;
document.getElementById("a2").value=b;
}
</script>
</body>