krahets/hello-algo · error · Error

Queue is empty

Error message

Queue is empty

What it means

Thrown by ArrayQueue.peek when the queue is empty. peek returns nums[front]; with queSize === 0 the front pointer points at no logical element, so the method throws. pop calls peek first and re-throws this error, so popping an empty queue surfaces the same message.

Source

Thrown at en/codes/typescript/chapter_stack_and_queue/array_queue.ts:58

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

    /* Dequeue */
    pop(): number {
        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(): number {
        if (this.isEmpty()) throw new Error('Queue is empty');
        return this.nums[this.front];
    }

    /* Return Array */
    toArray(): number[] {
        // 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 isEmpty() (or size() === 0) before pop/peek.
2. Drain with while (!queue.isEmpty()).
3. Wrap pop in a helper returning undefined on empty.

Example fix

// before
const v = queue.pop(); // throws if empty

// after
const v = queue.isEmpty() ? undefined : queue.pop();

const v = queue.isEmpty() ? undefined : queue.pop();

function hasElements(q) { return typeof q.isEmpty === 'function' && !q.isEmpty(); }

try { return queue.pop(); }
catch (e) { if (!/Queue is empty/.test(e.message)) throw e; return undefined; }

When it happens

Trigger: Calling pop or peek before any push; dequeueing more items than were enqueued; concurrent producers/consumers where the consumer outruns production.

Common situations: Polling loops without an emptiness check; queue drained then peeked again; capacity/front desync after failed operations.

Related errors

• Queue is empty
• 队列为空
• 队列为空
• Heap is empty.
• The Deque Is Empty.

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/34c708b82a9b5b06. Report an issue: GitHub.