krahets/hello-algo · error · Error

The Deque Is Empty.

Error message

The Deque Is Empty.

What it means

Thrown by ArrayDeque.peekFirst() ('The Deque Is Empty.') when the deque holds zero elements. peekFirst reads nums[front]; on an empty deque front points at stale/uninitialized storage, so the guard prevents returning garbage. popFirst() delegates to peekFirst(), so it also surfaces here.

Source

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

    /* 队首出队 */
    popFirst(): number {
        const num: number = this.peekFirst();
        // 队首指针向后移动一位
        this.front = this.index(this.front + 1);
        this.queSize--;
        return num;
    }

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

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

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

    /* 返回数组用于打印 */
    toArray(): number[] {
        // 仅转换有效长度范围内的列表元素
        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. Guard with isEmpty(): if (!deque.isEmpty()) deque.peekFirst().
2. Loop with while (!deque.isEmpty()).
3. Track element count externally and never read the front at zero.

Example fix

// before
const head = deque.peekFirst(); // throws when empty
// after
const head = deque.isEmpty() ? undefined : deque.peekFirst();

function safePeekFirst(deque) {
  return deque.isEmpty() ? undefined : deque.peekFirst();
}

null

null

When it happens

Trigger: Calling peekFirst() or popFirst() when queSize === 0; draining the deque in a loop without an emptiness check.

Common situations: Using the deque as a queue/stack and reading the front after it was drained; off-by-one in a consumer loop; calling peek before any push.

Related errors

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

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/61b2ec9da319fb4b. Report an issue: GitHub.