krahets/hello-algo · error · Error

The Deque Is Empty.

Error message

The Deque Is Empty.

What it means

Thrown by peekFirst() on an array-backed deque when the deque is empty. peekFirst() reads the front element without removing it; popFirst() delegates to it. The guard prevents reading this.#nums[this.#front] on an empty structure where the index is meaningless.

Source

Thrown at en/codes/javascript/chapter_stack_and_queue/array_deque.js:88

    /* Rear of the queue dequeue */
    popFirst() {
        const num = 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() {
        const num = this.peekLast();
        this.#queSize--;
        return num;
    }

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

    /* Driver Code */
    peekLast() {
        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() {
        // Elements enqueue
        const res = [];
        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() before peekFirst()/popFirst().
2. In a consume loop use while (!deque.isEmpty()) { const v = deque.popFirst(); ... }.
3. Return a sentinel/Option from a wrapper if empty-peek is a valid application state.
4. Track enqueue/dequeue counts if you need to bound a fixed number of operations.

Example fix

// before
const front = deque.peekFirst(); // throws when empty

// after
const front = deque.isEmpty() ? null : deque.peekFirst();

if (!deque.isEmpty()) {
  const front = deque.peekFirst();
} else {
  // handle empty deque
}

function dequeHasFront(deque) {
  return typeof deque.isEmpty === 'function' && !deque.isEmpty();
}

try {
  const front = deque.peekFirst();
} catch (e) {
  if (e.message === 'The Deque Is Empty.') { /* empty */ }
  else throw e;
}

When it happens

Trigger: Calling peekFirst() or popFirst() on an empty deque; reading the front before any push; dequeuing more than was enqueued.

Common situations: BFS/level-order traversal peeking at empty frontier; producer-consumer where consumer races ahead; calling popFirst in a loop without an emptiness guard.

Related errors

• The Deque Is Empty.
• The Deque Is Empty.
• 队列为空
• Queue is empty
• キューが空です

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/ce531b7890502a08. Report an issue: GitHub.