Вызов функции один раз

const once = fn => {
  let called = false;
  return function(…args) {
    if (called) return;
    called = true;
    return fn.apply(this, args);
  };
};

const startApp = function(event) {
  console.log(this, event); // document.body, MouseEvent
};
document.body.addEventListener('click', once(startApp));
// only runs `startApp` once upon click