JS to toggle data attributes from: https://toddmotto.com/stop-toggling-classes-with-js-use-behaviour-driven-dom-manipulation-with-data-states/

toggle.js

var nav = document.querySelector('.nav__toggle');
var toggleState = function (elem, one, two) {
  var elem = document.querySelector(elem);
  elem.setAttribute('data-state', elem.getAttribute('data-state') === one ? two : one);
};

nav.onclick = function (e) {
  toggleState('.nav ul', 'closed', 'open');
  e.preventDefault();
};

// ES5 using .bind() #ftw
nav.addEventListener('click', toggleState.bind(null, '.nav ul', 'closed', 'open'), false);