如果我没猜错的话，你想通过比较effective_date和book_date得到乘数，然后你想计算该汇率乘数的交易次数乘数的值。
例如，如果book_date为：2020-04-01

• 如果存在有效日期早于2020-04-1的汇率，则该汇率就是我们要查找的汇率。如果存在多个有效日期，则需要最新的汇率。

如果我说错了请纠正我。
我创建并测试了这个函数，它似乎工作：

const multiplyByTransaction = (transaction) => {
  let multiplier = null;

  // sort in ascending order in case they are not sorted.
  // you can remove the sort() if you have them sorted already
  const sorted_rates = monthly_rates.sort(
    (a, b) => a.effective_date - b.effective_date
  );

  for (let index = 0; index < sorted_rates.length; index++) {
    if (
      Date.parse(transaction.book_date) <
      Date.parse(sorted_rates[index].effective_date)
    ) {
      multiplier = sorted_rates[index > 0 ? --index : index].multiplier;
      break;
    }
  }

  // this means that there isn't any effective_date that is greater than book_date of the transaction
  // so we assign the latest rate's multiplier to our multiplier
  if (!multiplier) {
    multiplier = sorted_rates[sorted_rates.length - 1].multiplier;
  }

console.log(
    `Calculating: Multiplier => ${multiplier} * value => ${
      transaction.value
    } = ${multiplier * transaction.value}`
  );
  return multiplier * transaction.value;
};

multiplyByTransaction(transactions[1]) // will log: Calculating: Multiplier => 1.3 * value => 192842 = 250694.6

希望能有所帮助！