String Length function in JavaScript

var my_str="Hello world!"
document.write(my_str.length)// 12

DEMO: Counting data entered by user

<script type="text/javascript">
function check_text1_length(){
var t1=document.getElementById("text1").value;
document.getElementById("msg").innerHTML="The length of data = " + t1.length;
}
</script>

<input type=text name=text1 id=text1>
<input type=button onclick=check_text1_length(); value=Check>
<div id=msg></div>