Paging.js

const hasNextChapter = true;
const hasNextPage = true;
const hasPreviousChapter = true;
const hasPreviousPage = true;

const goToNextChapter = function() {};
const goToNextPage = function() {};
const goToPreviousChapter = function() {};
const goToPreviousPage = function() {};

let down = {};

$(window).on("keyup", function(e) {
  const code = e.keyCode ? e.keyCode : e.which; //which code?
  return delete down[code];
});

$(window).on("keydown", function(e) {
  const code = e.keyCode ? e.keyCode : e.which; //which code?
  if (down[code]) {
    return;
  }
  down[code] = true;
  if (code === 39 || code === 68 || code === 37 || code === 65 || code === 40 || code === 83 || code === 38 || code === 87) {
    e.preventDefault();
  }
  switch (code) {
    case 39:
    case 68: // next (right arrow, d)
      if (e.shiftKey && hasNextChapter) {
        return goToNextChapter();
      } else if (!e.shiftKey && hasNextPage) {
        return goToNextPage();
      }
      break;
    case 37:
    case 65: // next (left arrow, a)
      if (e.shiftKey && hasPreviousChapter) {
        return goToPreviousChapter();
      } else if (!e.shiftKey && hasPreviousPage) {
        return goToPreviousPage();
      }
      break;
    case 40:
    case 83: // down (down arrow, s)
      return $("html, body").animate({
        scrollTop: '+=400'
      }, 150, 'swing', function() {
        return delete down[code];
      });
    case 38:
    case 87: // up (up arrow, w)
      return $("html, body").animate({
        scrollTop: '-=400'
      }, 150, 'swing', function() {
        return delete down[code];
      });
  }
});