var xhrRoom = new XMLHttpRequest(); //step 1:
xhrRoom.onreadystatechange = function () {
  if(xhrRoom.readyState === 4 && xhrRoom.status === 200) {
        console.log(xhrRoom.responseText);
    var rooms = JSON.parse(xhrRoom.responseText);
    var roomsHTML ='<ul class="rooms">';

    for (var i=0; rooms.length; i+=1){
      if (rooms[i].available === true){
        roomsHTML += '<li class="empty">';
      }else {
        roomsHTML += '<li class="full">';
      }
      roomsHTML += rooms[i].room;
      roomsHTML += '</li>';

    }
    statusHTML += '</ul>';
    document.getElementById('roomList').innerHTML = roomsHTML;
  } // step 2:
};
xhrRoom.open('GET','../data/rooms.json'); //step 3
xhrRoom.send(); // step 4

console.log(xhrRoom.responseText);