function myFunction() {
    var x = document.getElementById("myTopnav");
    if (x.className === "topnav") {
      x.className += " responsive";
    } else {
      x.className = "topnav";
    }
  }



var demoButtons;

function start () {
  
  // Add event "click" to "demo buttons"
  demoButtons = document.querySelectorAll ('.js-modify');
  for (var i = 0; i < demoButtons.length; i++) {
    demoButtons[i].addEventListener ('click', toggleEffect);
  }
  
  // Add event "click" to "save buttons"
  var saveButtons = document.querySelectorAll ('.js-save');
  for (var i = 0; i < saveButtons.length; i++) {
    saveButtons[i].addEventListener ('click', toggleActive);
  }
  
}

// Toggle "effect" classes
function toggleEffect () {
  var target = document.querySelector (this.dataset.target);
      target.dataset.effect = this.dataset.effect;
  
  for (var i= 0; i < demoButtons.length; i++) {
    demoButtons[i].classList.remove ('active');
  }
  
  toggleActive.call (this);
}

// Toggle "active" class
function toggleActive () {
  this.classList.toggle ('active');
}

// Invoke "start ()" function
window.addEventListener ('load', start);