Creating "Back to Top" button in WordPress.

back-to-top.js

jQuery( document ).ready(function($){
  var offset = 100,
      speed = 250,
      duration = 500,
      scrollButton = $('#topbutton');
  
  $( window ).scroll( function() {
    if ( $( this ).scrollTop() < offset) {
      scrollButton.fadeOut( duration );
    } else {
      scrollButton.fadeIn( duration );
    }
  });
  
  scrollButton.on( 'click', function(e){
    e.preventDefault();
    $( 'html, body' ).animate({
      scrollTop: 0
    }, speed);
  });
});

functions.php

/**
 * Enqueue button javascript script.
 */
function themeslug_add_button_script() {
  wp_enqueue_script( 'custom-script', get_stylesheet_directory_uri() . '/js/back-to-top.js', array( 'jquery' ) );
}
add_action( 'wp_enqueue_scripts', 'themeslug_add_button_script' );

/**
 * Add button HTML to the footer section.
 */
function themeslug_add_scroll_button() {
  echo '<a href="#" id="topbutton"></a>';
}
add_action( 'wp_footer', 'themeslug_add_scroll_button' );

style.css

#topbutton {
  position: fixed;
  display: none;
  height: 40px;
  width: 40px;
  line-height: 40px;
  right: 15px;
  bottom: 15px;
  z-index: 1;
  background: #000000;
  border-radius: 2px;
  text-decoration: none;
  color: #ffffff;
  text-align: center;
}

#topbutton:after {
  content: "\2191";
}