var header = document.getElementById("header");
var hamburgerMenu = document.getElementById("header-hamburger");
var nav = document.getElementById("header-nav");
header.className += " header-js";
var className = header.className;
var oldHamburgerTabIndex = hamburgerMenu.tabIndex;

function toggleHamburger() {
  if (header.className.match(/(?:^| )header-nav-opened( |$)/)) {
    header.className = className;
    document.documentElement.style.overflow = null;
    hamburgerMenu.tabIndex = oldHamburgerTabIndex;
  } else {
    header.className += " header-nav-opened";
    document.documentElement.style.overflow = "hidden";
    hamburgerMenu.tabIndex = 1;
  }
}

hamburgerMenu.onclick = toggleHamburger;
hamburgerMenu.onkeydown = function (e) {
  if (e.key == "Enter") toggleHamburger();
}