var stack = [];
var lastState = document.getElementById('drop_zone_content');
stack.push(lastState);
var observer = new MutationObserver(function(div) {

if(current_hash != old_hash){

var newState = document.getElementById('drop_zone_content');
stack.push(newState);
old_hash = current_hash;

}
});

function undo(){

document.getElementById('drop_zone_content').remove();
lastState = stack.pop();
var returnBack = document.getElementById('uploadFiles');
returnBack.insertBefore(lastState, null);
}