hide nav bar dont hide when scrolling and i want it toggle when i scroll by javascript

Question

I have typed every thing right but does not work.

var prevScrollpos = window.pageYOffset;
window.onscroll = function () {
  var currentScrollpos = window.pageYOffset;
  if (prevScrollpos > currentScrollpos) {
    document.getElementsByClassName("nav").style.top = "0";
  } else {
    document.getElementsByClassName("nav").style.top = "-100px";
  }
  prevScrollpos = currentScrollpos;
};

Answer 1

Please try again this code. Should be document.getElementsByClassName("nav")[0]

    var prevScrollpos = window.pageYOffset;
    window.onscroll = function () {
      var currentScrollpos = window.pageYOffset;
      if (prevScrollpos > currentScrollpos) {
        document.getElementsByClassName("nav")[0].style.top = "0";
      } else {
        document.getElementsByClassName("nav")[0].style.top = "-100px";
      }
      prevScrollpos = currentScrollpos;
    };