JS - active class on navigation

Question

  var nav = document.getElementById("nav");
  var menu = header.getElementsByClassName("menu");
  for (var i = 0; i < menu.length; i++) {
    menu[i].addEventListener("click", function() {
    var current = document.getElementsByClassName("active");
    current[0].className = current[0].className.replace(" active", "");
    this.className += " active";
    });
  }