[Vlasenko Fedor]

<div id='first'>first</div>
<div id='second' style="display: none">second</div>
<a id="back">back</a>
<a id="forward">forward</a>
<script>
  Array.apply(null, document.querySelectorAll('#back, #forward')).map(function (element) {
    element.addEventListener("click", navigation);
  });
  function navigation() {
    var style = {
      back: ["block", "none"],
      forward: ["none", "block"]
    }[this.id];
    first.style.display = style[0];
    second.style.display = style[1];
  }
</script>
:)