krahets/hello-algo · error · Error

The Deque Is Empty.

Error message

The Deque Is Empty.

What it means

An Error 'The Deque Is Empty.' thrown by peekFirst() in ArrayDeque (array_deque.ts:88). peekFirst() returns nums[front], which is meaningless when queSize === 0 because front points at stale or uninitialized data. Both pop methods (popFirst/popLast) call a peek first, so this guard also protects pops from operating on an empty ring buffer.

Source

Thrown at zh-hant/codes/typescript/chapter_stack_and_queue/array_deque.ts:88

    /* 佇列首出列 */
    popFirst(): number {
        const num: number = this.peekFirst();
        // 佇列首指標向後移動一位
        this.front = this.index(this.front + 1);
        this.queSize--;
        return num;
    }

    /* 佇列尾出列 */
    popLast(): number {
        const num: number = this.peekLast();
        this.queSize--;
        return num;
    }

    /* 訪問佇列首元素 */
    peekFirst(): number {
        if (this.isEmpty()) throw new Error('The Deque Is Empty.');
        return this.nums[this.front];
    }

    /* 訪問佇列尾元素 */
    peekLast(): number {
        if (this.isEmpty()) throw new Error('The Deque Is Empty.');
        // 計算尾元素索引
        const last = this.index(this.front + this.queSize - 1);
        return this.nums[last];
    }

    /* 返回陣列用於列印 */
    toArray(): number[] {
        // 僅轉換有效長度範圍內的串列元素
        const res: number[] = [];
        for (let i = 0, j = this.front; i < this.queSize; i++, j++) {
            res[i] = this.nums[this.index(j)];
        }

View on GitHub (pinned to 69932aed18)

Solutions

1. Check deque.isEmpty() (or deque.size() === 0) before peekFirst/popFirst.
2. Loop with `while (!deque.isEmpty())` when draining.
3. Remember capacity() is the backing array length, not the element count — use size() for the latter.
4. Keep push and pop counts balanced, or cache the count you intend to consume.

Example fix

// before: peeking an empty deque throws
const head = deque.peekFirst();

// after: guard with emptiness check
const head = deque.isEmpty() ? undefined : deque.peekFirst();

// Guard the head end of an ArrayDeque
function safePeekFirst(deque: ArrayDeque): number | undefined {
    return deque.isEmpty() ? undefined : deque.peekFirst();
}
if (!deque.isEmpty()) {
    const head = deque.popFirst();
}

const nonEmpty = (deque: ArrayDeque): boolean => !deque.isEmpty();

try {
    const head = deque.peekFirst();
} catch (e) {
    if (e instanceof Error && /Deque Is Empty/.test(e.message)) {
        // deque drained; handle empty state
    } else throw e;
}

When it happens

Trigger: Calling deque.peekFirst() or deque.popFirst() when deque.isEmpty() is true (queSize === 0). Popping more elements than were pushed, or peeking a deque that was just constructed with `new ArrayDeque(capacity)` and never filled.

Common situations: Unbalanced push/pop counts in a ring-buffer deque; assuming a peek is safe because capacity > 0 (capacity is storage size, not element count); draining the deque in a loop without an emptiness check; calling popFirst after popLast already emptied it.

Related errors

• The Deque Is Empty.
• The Deque Is Empty.
• The Deque Is Empty.
• The Deque Is Empty.
• Heap is empty.

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/45af093ad5a0ff23. Report an issue: GitHub.