krahets/hello-algo · error · Error

The Deque Is Empty.

Error message

The Deque Is Empty.

What it means

Thrown by ArrayDeque.peekFirst when the deque is empty. peekFirst reads nums[front]; with queSize === 0 there is no valid element and the index would be stale, so the method throws rather than return garbage. popFirst delegates here, so it propagates the same error.

Solutions

1. Check isEmpty() before peekFirst/popFirst.
2. Drain with while (!deque.isEmpty()).
3. Wrap in a helper that returns undefined on empty.

Example fix

// before
const head = deque.peekFirst(); // throws if empty

// after
const head = deque.isEmpty() ? undefined : deque.peekFirst();

const head = deque.isEmpty() ? undefined : deque.peekFirst();

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

try { return deque.peekFirst(); }
catch (e) { if (!/Deque Is Empty/.test(e.message)) throw e; return undefined; }

When it happens

Trigger: Calling peekFirst or popFirst before any push; calling after the deque has been fully drained; using front/queSize pointers that desynced from actual inserts.

Common situations: Unconditional peek in polling loops; deque used as a work queue with no items queued; off-by-one in size bookkeeping.

Related errors

• Heap is empty.
• Queue is empty
• Queue is empty
• Stack is empty
• Stack is empty

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/0baeddae7a95a65b. Report an issue: GitHub.

Appendix: source

Thrown at en/codes/typescript/chapter_stack_and_queue/array_deque.ts:88

    /* Rear of the queue dequeue */
    popFirst(): number {
        const num: number = this.peekFirst();
        // Move front pointer backward by one position
        this.front = this.index(this.front + 1);
        this.queSize--;
        return num;
    }

    /* Access rear of the queue element */
    popLast(): number {
        const num: number = this.peekLast();
        this.queSize--;
        return num;
    }

    /* Return list for printing */
    peekFirst(): number {
        if (this.isEmpty()) throw new Error('The Deque Is Empty.');
        return this.nums[this.front];
    }

    /* Driver Code */
    peekLast(): number {
        if (this.isEmpty()) throw new Error('The Deque Is Empty.');
        // Initialize double-ended queue
        const last = this.index(this.front + this.queSize - 1);
        return this.nums[last];
    }

    /* Return array for printing */
    toArray(): number[] {
        // Elements enqueue
        const res: number[] = [];
        for (let i = 0, j = this.front; i < this.queSize; i++, j++) {
            res[i] = this.nums[this.index(j)];
        }

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