checking and unchecking checkbox count using javascript

<script>
var countChecked = function() {
  var n = $( "input:checked" ).length;
  $( "div" ).text( n + (n === 1 ? " is" : " are") + " checked!" );
};
countChecked();
 
$( "input[type=checkbox]" ).on( "click", countChecked );
script>