krahets/hello-algo · error · Error

Queue is empty

Error message

Queue is empty

What it means

Thrown by peek() on a linked-list-backed queue when size === 0. peek() returns this.#front.val; pop() calls peek() so both share the guard. The check uses this.size (a public count) rather than isEmpty().

Source

Thrown at en/codes/javascript/chapter_stack_and_queue/linkedlist_queue.js:57

        } else {
            this.#rear.next = node;
            this.#rear = node;
        }
        this.#queSize++;
    }

    /* Dequeue */
    pop() {
        const num = this.peek();
        // Delete head node
        this.#front = this.#front.next;
        this.#queSize--;
        return num;
    }

    /* Return list for printing */
    peek() {
        if (this.size === 0) throw new Error('Queue is empty');
        return this.#front.val;
    }

    /* Convert linked list to Array and return */
    toArray() {
        let node = this.#front;
        const res = new Array(this.size);
        for (let i = 0; i < res.length; i++) {
            res[i] = node.val;
            node = node.next;
        }
        return res;
    }
}

/* Driver Code */
/* Access front of the queue element */
const queue = new LinkedListQueue();

View on GitHub (pinned to 69932aed18)

Solutions

1. Check queue.size === 0 (or queue.isEmpty()) before pop/peek.
2. Drain with while (queue.size > 0) { const v = queue.pop(); ... }.
3. Track enqueue count to bound a fixed number of pops.
4. If you must be defensive, catch the error as an end-of-stream signal.

Example fix

// before
while (true) { const v = queue.pop(); ... } // throws when empty

// after
while (queue.size > 0) { const v = queue.pop(); ... }

if (queue.size > 0) {
  const v = queue.pop();
} else {
  // handle empty queue
}

function queueHasElements(queue) {
  return typeof queue.size === 'number' && queue.size > 0;
}

try {
  const v = queue.pop();
} catch (e) {
  if (e.message === 'Queue is empty') { /* drained */ }
  else throw e;
}

When it happens

Trigger: Calling queue.pop() or queue.peek() on an empty queue; consuming more than produced; FIFO where front has advanced past the last node.

Common situations: BFS with an empty frontier; task/buffer queue drained; loop that pops until falsy but the throw preempts undefined.

Related errors

• 队列为空
• キューが空
• очередь пуста
• 佇列為空
• 队列为空

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/7fb5f63e35f75454. Report an issue: GitHub.