window.onload = function slider () {
var img = document.querySelectorAll('#gallery-item img');
var wrapSpan = document.getElementById('wrap-span');
for(var i = 0; i < img.length; i++) {
var addSpan = document.createElement('span');
addSpan.classList.add('gallery-span-style');
wrapSpan.appendChild(addSpan);
};
var listSpan = document.getElementsByClassName('gallery-span-style');
for(var i = 0; i < listSpan.length; i++) {
var index = listSpan[i];
listSpan[i].onclick = function () {
img[index].style.display = 'block';
}
}
};