I get category id by js and want to use it to get category description. when i am passing this id using ajax into php variable its print correct output but when i try to put this id in get_description code ajax give 500 error and not return output why this happen please help me.

Below is my code.

<script type="text/javascript">
 $(".-filter").click(function() {
        var js_var = this.id;

               $.ajax ({
                    type: "POST",
                    url: "<?php echo plugin_dir_url( __FILE__ ); ?>category.php",
                    data: { val : js_var },
                    success: function( result    ) {               
                   $("#update").html(result);
            }

        });
    });
</script>

<div id="update">
<?php 
$cat_id = $_POST['val'];
echo $cat_id;
//echo term_description($cat_id,'category');
?>`enter code here
</div>

Thanks,