Javascript Upper case and lower case and reverse example

<script>
function check(){
var nu=document.getElementById('txtNumber').value;
var v=document.getElementsByTagName("p")[0];
v.innerHTML="";
if(nu.length<9){
v.innerHTML="Please enter min 9 letters";
return;
}
if(nu.length>3){
for(var i=0; i<3; i++){
v.innerHTML+=nu[i].toLowerCase();
}
for(var i=nu.length-4; i>2; i--){
v.innerHTML+=nu[i];
}
for(var i=nu.length-3; i<nu.length; i++){
v.innerHTML+=nu[i].toUpperCase();
}
}
}
</script>
<input id="txtNumber">
<p></p>
<button onclick="check()">show</button>