var btn_start = document.querySelector('#btn-start');
var result = document.querySelector('#result');


function wait_for_user_input() {
  var dlg = document.querySelector('#prompt');
  var input = dlg.querySelector('input');
  var done = dlg.querySelector('button');
  dlg.style.display = 'block';
  input.value = '';

  return new Promise(function(resolve, reject) {
    done.addEventListener('click', function() {
      resolve(input.value);
      dlg.style.display = 'none';
    });
  });
}

btn_start.addEventListener('click', function() {
  wait_for_user_input().then(function(name) {
    result.innerHTML = 'Hello! ' + name;
  });
});