krahets/hello-algo · error · Error

Queue is empty

Error message

Queue is empty

What it means

Thrown by peek() on an array-backed circular queue when empty. peek() returns this.#nums[this.#front]; pop() calls peek() first so pop also surfaces this error. The guard prevents returning undefined from an empty slot and keeps the front pointer arithmetic honest.

Source

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

        // Add num to the rear of the queue
        const rear = (this.#front + this.size) % this.capacity;
        // Front pointer moves one position backward
        this.#nums[rear] = num;
        this.#queSize++;
    }

    /* Dequeue */
    pop() {
        const num = this.peek();
        // Move front pointer backward by one position, if it passes the tail, return to array head
        this.#front = (this.#front + 1) % this.capacity;
        this.#queSize--;
        return num;
    }

    /* Return list for printing */
    peek() {
        if (this.isEmpty()) throw new Error('Queue is empty');
        return this.#nums[this.#front];
    }

    /* Return Array */
    toArray() {
        // Elements enqueue
        const arr = new Array(this.size);
        for (let i = 0, j = this.#front; i < this.size; i++, j++) {
            arr[i] = this.#nums[j % this.capacity];
        }
        return arr;
    }
}

/* Driver Code */
/* Access front of the queue element */
const capacity = 10;
const queue = new ArrayQueue(capacity);

View on GitHub (pinned to 69932aed18)

Solutions

1. Check queue.isEmpty() (or queue.size === 0) before pop/peek.
2. Use while (!queue.isEmpty()) { const v = queue.pop(); ... } for draining.
3. Track enqueue count to bound a fixed number of pops.
4. If pop must be defensive, catch the error and treat as end-of-stream.

Example fix

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

// after
while (!queue.isEmpty()) { const v = queue.pop(); ... }

if (!queue.isEmpty()) {
  const v = queue.pop();
} else {
  // handle empty queue
}

function queueHasElements(queue) {
  return typeof queue.isEmpty === 'function' && !queue.isEmpty();
}

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; dequeuing more than was enqueued; FIFO processing where the consumer outpaces the producer.

Common situations: BFS frontier drained; message/buffer queue consumed faster than filled; loop that pops until falsy (but throw interrupts before undefined).

Related errors

• 队列为空
• キューが空です
• The Deque Is Empty.
• 队列为空
• The Deque Is Empty.

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/e94bc4d5872451ab. Report an issue: GitHub.