循环中的异步调用延迟

Question

I have a function which makes two asynchronous calls to the database inside a loop. The issue is that the return function works before the data from the loop is retrieved.

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

You're making all of those calls in parallel by doing them in the map callback. If you really want to do that, you need to wait for those calls to settle by using Promise.all :

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

If you want to do them in series instead, use a for loop and await each response. :-) But it looks like parallel is okay.

Answer 2

You need to await the map and then it will wait for it otherwise being async doesn't make it wait it actually means it will go on to the next code unless you tell it to 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.