tabs.js

document.querySelectorAll("#nav li").forEach(function(navEl) {
  navEl.onclick = function() { toggleTab(this.id, this.dataset.target); }
});

function toggleTab(selectedNav, targetId) {
  var navEls = document.querySelectorAll("#nav li");

  navEls.forEach(function(navEl) {
    if (navEl.id == selectedNav) {
      navEl.classList.add("is-active");
    } else {
      if (navEl.classList.contains("is-active")) {
        navEl.classList.remove("is-active");
      }
    }
  });

  var tabs = document.querySelectorAll(".tab-pane");

  tabs.forEach(function(tab) {
    if (tab.id == targetId) {
      tab.style.display = "block";
    } else {
      tab.style.display = "none";
    }
  });
}