Callbacks & Callback Hell

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

Prevents memory leaks
Ensures the callback is only called once
The once utility ensures a callback is only executed once: 1) Prevents multiple executions of the same callback, 2) Useful for cleanup operations or one-time initializations, 3) Maintains the original this context, 4) Thread-safe for synchronous operations, 5) Common in event handling where multiple triggers should only execute once, 6) Important for preventing duplicate side effects.
Handles errors automatically
Makes callbacks async