Question

I am trying to query, that would get all of the earning and withdrawal amount and sum, but somehow after the forEach loop is done and exits, all of the updated values I placed in a variable return to their original values.

  var withdrawbalance = 0;
  var totalearning = 0;

  userplans.forEach((up) => {
    DepositEarning.findOne({deposit: up.deposit._id})
      .then(depositearning => {
        withdrawbalance += parseInt(depositearning.WithdrawableBalance, 10);
      });
    Earning.findOne({deposit: up.deposit._id})
      .then(earnings => {
        totalearning += parseInt(earnings.Earning, 10);
    });
  })

  console.log(withdrawbalance);
  console.log(totalearning);

Answer 1

您正在forEach中运行异步代码，并且forEach不会等到异步代码完成，为此，必须用等待的原语包装异步代码，像这样

await Promise.all(userplans.map(async (up) => {
    await DepositEarning.findOne({deposit: up.deposit._id})
      .then(depositearning => {
        withdrawbalance += parseInt(depositearning.WithdrawableBalance, 10);
      });
    await Earning.findOne({deposit: up.deposit._id})
      .then(earnings => {
        totalearning += parseInt(earnings.Earning, 10);
    });
  }))

Answer 2

我认为原因可能是DepositEarning.findOne（）和Earning.findOne（）都是异步函数。这意味着它们不会立即返回值，因此循环后您的变量仍然相同。

Answer 3

这里是一种实用的方法，可消除循环内部的突变副作用。为此，您需要一个支持Object Rest传播的引擎：

async function calculate(userplans) {
    const add = (acc, n) => acc + n;
    const flatten = (acc, e) => ({
        depositEarnings: [...acc.depositEarnings, e.depositEarnings], 
        earnings: [...acc.earnings, e.earnings]
    });

    const {depositEarnings, earnings} = (await Promise.all(userplans
          .map(async (up) => ({
              depositEarnings: await DepositEarning.findOne({deposit: up.deposit._id}),
              earnings: await Earning.findOne({deposit: up.deposit._id})
}))))
          .reduce(flatten, {depositEarnings: [], earnings: []});

    const withdrawalBalance = depositEarnings
          .map(d => parseInt(d.WithdrawableBalance, 10))
          .reduce(add, 0);
    const totalEarning = earnings
          .map(e => parseInt(e.Earning, 10))
          .reduce(add, 0);

    console.log(withdrawalBalance);
    console.log(totalEarning);
}

Answer 4

只需加上我的两分钱，

正如其他答案所述，您的foreach循环正在使用异步代码。它正在使用不属于Js的某些API。

将调用推送到调用堆栈，然后将其从调用堆栈中删除。因为现在该工作将由API处理。呼叫

console.log(withdrawbalance); console.log(totalearning);

被添加到调用堆栈中，现在的值是什么？ 0 0。

API完成工作后，将在task queue中添加函数，然后event loop将检查调用堆栈是否为空。由于现在为空，因此将函数添加到调用堆栈中。

然后运行。然后您将获得withdrawbalance and totalearning值的真实值。

Answer 5

forEach is asynchronous. So the console.log runs before your mutation loop. You can make it synchronous by using map, but even then - you have asynchronous calls inside it. You will need to flatten it further - probably by doing Promise.all on a map, then reducing the output of that.

The variables I updated in my forEach, return to their original value

5 个答案: