Add toTop Button with WordPress Hook

toTop_button.php

<?php

// Add scripts to wp_footer()
function totop_script() { ?>
<script>
jQuery(function ($) {
  var $window = $(window);
  var $buttonTop = $('.button-top');
  var scrollTimer;

  $buttonTop.on('click', function () {
    $('html, body').animate({
      scrollTop: 0,
    }, 400);
  });

  $window.on('scroll', function () {
    clearTimeout(scrollTimer);
    scrollTimer = setTimeout(function() {
     if ($window.scrollTop() > 100) {
        $buttonTop.addClass('button-top-visible');
      } else {
        $buttonTop.removeClass('button-top-visible');
      }         
    }, 250);
  });  
})
</script>
	<button class="button-top">↑</button>
<?php }
add_action( 'wp_footer', 'totop_script' );