对象中的项如何从一个函数转移到另一个函数?

我正在学习react redux,我正在使用 firebase 用于存储数据。

我安装了thunk中间件。一切都正常,我只是不明白为什么。

据我所知,const expense是另一个函数范围内的一个对象。怎么可能 addExpense 获取访问权限?

export const addExpense = (expense) => ({
    type: 'ADD_EXPENSE',
    expense
});

export const startAddExpense = (expenseData = {}) => {
    return (dispatch) => {
        const {
            description = '',
            note = '',
            amount = 0,
            createdAt = 0
        } = expenseData;
        const expense = { description, note, amount, createdAt };

        database.ref('expenses').push(expense).then((ref) => {
            dispatch(addExpense({
                id: ref.key,
                ...expense
            }));
        });
    };
};