<div id="fullpage">
  <div class="section">Some section 1</div>
  <div class="section">Some section 2</div>
  <div class="section">Some section 3</div>
  <div class="section">Some section 4</div>
</div>

// const afterLoadFullpage = new CustomEvent('afterLoadFullpage', );

var myFullpage = new fullpage('#fullpage', {
  //options here
  autoScrolling:true,
  // afterLoad: function(origin, destination, direction) {
  //    let anniversaryText = document.querySelector('.anniversary-text-wrap');
  //    if (destination.index == 1) {
  //      // тут работает
  //      // alert('section 2');
  //    }
  //  },
  afterLoad: function(origin, destination, direction) {
      document.dispatchEvent(new CustomEvent("afterLoad", {
        detail: { origin, destination, direction }
      }));
    
      if (destination.index == 1) {
        // тут работает
        // alert('section 2');
        // afterLoad: () => customEmitter.emit('afterLoad', destination),
        // document.dispatchEvent(afterLoadFullpage);
      }
    // afterLoad: () => customEmitter.emit('afterLoad', ...arguments),
    },
});


// myFullpage.on('afterLoad', function(origin, destination, direction) {
//  if (destination.index == 1) {
//    // а тут нет
    // alert('section 2');
//  }
// })

// customEmitter.on('event', doWhatever);

document.addEventListener('afterLoad', function(e) {
  alert(e.detail.destination.index);
})