循环中的异步调用延迟

Question

我有一个函数可以在一个循环中对数据库进行两次异步调用。问题是 return 函数在从循环中检索数据之前起作用。

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

你需要 await 地图，然后它会等待它，否则异步不会让它等待它实际上意味着它将继续下一个代码，除非你告诉它 await。像这样： const customerList = await customers.map.... 现在因为 map 没有 return 承诺，你需要用 Promise.all 或单独的承诺将它包装在承诺中。

Answer 2

您可以通过在 map 回调中并行进行所有这些调用。如果你真的想这样做，你需要等待这些调用通过使用 Promise.all:

来解决

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

如果您想依次进行，请使用 for 循环并等待每个响应。 :-) 但看起来并行没问题。

循环中的异步调用延迟

Async calls in loop delayed

javascript

database

async-await

knex.js