krahets/hello-algo · error · Error

佇列為空

Error message

佇列為空

What it means

Thrown by ArrayQueue.peek() (message: '佇列為空' = 'queue is empty') when the queue has no elements. peek() returns nums[front]; without the guard it returns stale/undefined data. pop() delegates to peek(), so pop on an empty queue also triggers this.

Source

Thrown at zh-hant/codes/javascript/chapter_stack_and_queue/array_queue.js:57

        // 透過取餘操作實現 rear 越過陣列尾部後回到頭部
        const rear = (this.#front + this.size) % this.capacity;
        // 將 num 新增至佇列尾
        this.#nums[rear] = num;
        this.#queSize++;
    }

    /* 出列 */
    pop() {
        const num = this.peek();
        // 佇列首指標向後移動一位，若越過尾部，則返回到陣列頭部
        this.#front = (this.#front + 1) % this.capacity;
        this.#queSize--;
        return num;
    }

    /* 訪問佇列首元素 */
    peek() {
        if (this.isEmpty()) throw new Error('佇列為空');
        return this.#nums[this.#front];
    }

    /* 返回 Array */
    toArray() {
        // 僅轉換有效長度範圍內的串列元素
        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 */
/* 初始化佇列 */
const capacity = 10;
const queue = new ArrayQueue(capacity);

View on GitHub (pinned to 69932aed18)

Solutions

  1. Check queue.isEmpty() before pop() or peek().
  2. Use while (queue.size > 0) for drain loops.
  3. If peeking for display, guard with isEmpty() and return null or a placeholder.

Example fix

// before
const val = queue.pop(); // throws '佇列為空' if empty (pop calls peek)

// after
while (queue.size > 0) {
    const val = queue.pop();
}
Defensive patterns

Strategy: validation

Validate before calling

// peek() and pop() (which calls peek) both throw on empty
if (queue.size > 0) {
    const val = queue.pop();
}

Try / catch

try {
    const val = queue.pop();
} catch (e) {
    if (e.message === '佇列為空') {
        // queue is empty — handle underflow
    } else throw e;
}

Prevention

When it happens

Trigger: Calling pop() or peek() on an empty queue, or dequeuing more items than were enqueued.

Common situations: Consumer loop draining faster than producer; FIFO processing with no guard; stale front pointer after capacity reset.

Related errors

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/90110a30739f00b0. Report an issue: GitHub.