What causes "堆疊為空" in krahets/hello-algo?

Check stack.size > 0 (or !stack.isEmpty()) before pop() or peek().

krahets/hello-algo · error · Error

堆疊為空

Error message

堆疊為空

What it means

Thrown by LinkedListStack.peek() (message: '堆疊為空' = 'stack is empty') when stackPeek is null. peek() reads stackPeek.val; without the guard a TypeError would occur. pop() delegates to peek(), so popping an empty stack surfaces this error.

Source

Thrown at zh-hant/codes/javascript/chapter_stack_and_queue/linkedlist_stack.js:46

    /* 入堆疊 */
    push(num) {
        const node = new ListNode(num);
        node.next = this.#stackPeek;
        this.#stackPeek = node;
        this.#stkSize++;
    }

    /* 出堆疊 */
    pop() {
        const num = this.peek();
        this.#stackPeek = this.#stackPeek.next;
        this.#stkSize--;
        return num;
    }

    /* 訪問堆疊頂元素 */
    peek() {
        if (!this.#stackPeek) throw new Error('堆疊為空');
        return this.#stackPeek.val;
    }

    /* 將鏈結串列轉化為 Array 並返回 */
    toArray() {
        let node = this.#stackPeek;
        const res = new Array(this.size);
        for (let i = res.length - 1; i >= 0; i--) {
            res[i] = node.val;
            node = node.next;
        }
        return res;
    }
}

/* Driver Code */
/* 初始化堆疊 */
const stack = new LinkedListStack();

View on GitHub (pinned to 69932aed18)

Solutions

Check stack.size > 0 (or !stack.isEmpty()) before pop() or peek().
Ensure push/pop are balanced in expression evaluation and DFS.
Return null instead of calling peek() when the stack is empty.

Example fix

// before
const val = stack.pop(); // throws '堆疊為空' (pop calls peek)

// after
if (stack.size > 0) {
    const val = stack.pop();
}

Defensive patterns

Strategy: validation

Validate before calling

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

Try / catch

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

Prevention

Check stack.size > 0 before pop() or peek().
Ensure push/pop are balanced in DFS and expression evaluation.
Return null instead of peek() when the stack is empty.

When it happens

Trigger: Calling pop() or peek() when no nodes have been pushed, or after all nodes were popped.

Common situations: Unbalanced push/pop in algorithm code; backtracking past the sentinel; DFS that pops the last element then peeks.

Related errors

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/465ed327340994db. Report an issue: GitHub.