Wordpress template ajax callback

wp_ajax_callback_template.php

<?php

/**
 *  Template ajax callback
 */

add_action( 'wp_ajax_callback', 'callback' );
add_action( 'wp_ajax_nopriv_callback', 'callback' );
function callback($a) {
  $data = dosomething($a);
  $data = json_encode($data);
  wp_die($data);
}

add_action('wp_footer', 'template_ajax_call_js');
function template_ajax_call_js() {
  ob_start();?>
<script>
jQuery(function($) {

    // Template ajax call
      $.ajax({
        url: ajaxurl, // supply ajax url
        data: {
          'action': 'callback',
          'input' : 'somevalue'
        },
        success:function(data) {
          console.log(data);
        },
        error: function(errorThrown){
          alert('error');
          console.log(errorThrown);
        }
      });

});
</script>
  <?php
}