循环中的异步调用延迟

Question

我有一个 function 在循环内对数据库进行两次异步调用。 问题是返回 function 在检索来自循环的数据之前起作用。

const myFunc = async (customers) => {
  const customerList = customers.map(async (customer) => {
    const cashCollected = await knex('cash_collections')
      .sum('amount_collected as amount')
      .where('account_code', customer.code)
      .first();
    const orderValue = await knex('orders')
      .sum('discounted_price as amount')
      .where('customer_id', customer.id)
      .first();
    const customerData = {
      name: customer.name,
      outstandingBalance: (orderValue.amount - cashCollected.amount),
    };
    // This line works after console.log(customerList);
    console.log(customerData);
    return customerData;
  });
   // console and return works before data is retrieved 
   // (before console.log(customerData) is run)
  console.log(customerList);
  return customerList;
};

// Function is called in another place
myFunc()

Answer 1

您可以通过在map回调中并行执行所有这些调用。 如果您真的想这样做，则需要使用Promise.all等待这些呼叫解决：

const customerList = await Promise.all(customers.map(async (customer) => {
    // ...
}));

如果您想连续执行它们，请使用for循环并等待每个响应。 :-) 但看起来并行是可以的。

Answer 2

您需要await map ，然后它将等待它，否则异步不会使其等待它实际上意味着它将go 进入下一个代码，除非您告诉它await 。 like so: const customerList = await customers.map.... Now since map does not return a promise you would need to wrap it in a promise either with Promise.all or an individual promise.