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export function toggle(){
  var nav = $('.header_nav');
  //表示位置
  var navTop = nav.offset().top+500;
  //ナビゲーションの高さ（シャドウの分だけ足してます）
  var navHeight = nav.height()+10;
  var showFlag = false;
  nav.css('top', -navHeight+'px');
  //ナビゲーションの位置まできたら表示
  $(window).scroll(function () {
    var winTop = $(this).scrollTop();
    if (winTop >= navTop) {
      if (showFlag == false) {
        showFlag = true;
        nav
          .addClass('is-show')
          .stop().animate({'top' : '0px'}, 400);
      }
    } else if (winTop <= navTop) {
      if (showFlag) {
        showFlag = false;
        nav.stop().animate({'top' : -navHeight+'px'}, 400, function(){
          nav.removeClass('is-show');
        });
      }
    }
  });

}

<!-- /#nav -->
<div class="header_nav sp-hide">
  <p class="tag is-entry_opened">募集中</p>
  <p>2017 Summer Internship</p>
  <div class="header_entry"><a href=""><p>entry<i class="icon-entry_arrow_right"></i>
    </p></a></div>
</div>
<!-- /#nav -->