如何在Node.js中将Mongoose查询结果赋值给本地变量并外部打印？

Mongoose的find()是异步操作，你写的console.log(monthlyExpense)会在Expense.find()的回调函数执行前就运行，所以此时monthlyExpense还是初始的undefined，自然打印不出正确结果。

方案1：用async/await写同步风格的异步代码（推荐） #

把逻辑放在async函数里，用await等待每个查询完成，保证打印时变量已经被赋值：

// 比如在Express路由里，把处理函数声明为async
app.get('/monthly-stats', async (req, res, next) => {
  try {
    // 查月度订单数据并求和
    const orders = await CompleteOrders.find({
      CompleteDate: { $gte: startDate, $lte: endDate }
    });
    const monthlyOrders = lodash.sumBy(orders, o => o.Total);
    res.locals.income = monthlyOrders;

    // 查月度支出数据并求和
    const expenses = await Expense.find({
      ExpenseDate: { $gte: startDate, $lte: endDate }
    });
    const monthlyExpense = lodash.sumBy(expenses, o => o.Expense);
    res.locals.Exp = monthlyExpense;

    // 现在打印就能得到正确值
    console.log(monthlyExpense);
    // 顺便计算月度利润
    const Profit_month = monthlyOrders - monthlyExpense;
  } catch (err) {
    next(err);
  }
});

方案2：并行查询提升效率 #

如果两个查询互相不依赖，可以用Promise.all同时发起，节省等待时间：

app.get('/monthly-stats', (req, res, next) => {
  Promise.all([
    // 同时发起两个查询
    CompleteOrders.find({ CompleteDate: { $gte: startDate, $lte: endDate } }),
    Expense.find({ ExpenseDate: { $gte: startDate, $lte: endDate } })
  ])
  .then(([orders, expenses]) => {
    const monthlyOrders = lodash.sumBy(orders, o => o.Total);
    const monthlyExpense = lodash.sumBy(expenses, o => o.Expense);
    
    res.locals.income = monthlyOrders;
    res.locals.Exp = monthlyExpense;
    
    console.log(monthlyExpense);
    const Profit_month = monthlyOrders - monthlyExpense;
  })
  .catch(err => next(err));
});

方案3：让数据库直接求和（性能最优） #

没必要把所有订单/支出数据都查出来再在代码里求和，用Mongoose的聚合查询让数据库直接计算总和，减少数据传输量：

app.get('/monthly-stats', async (req, res, next) => {
  try {
    // 聚合查询订单总和
    const orderAggregate = await CompleteOrders.aggregate([
      { $match: { CompleteDate: { $gte: startDate, $lte: endDate } } },
      { $group: { _id: null, totalIncome: { $sum: "$Total" } } }
    ]);
    const monthlyOrders = orderAggregate[0]?.totalIncome || 0;
    res.locals.income = monthlyOrders;

    // 聚合查询支出总和
    const expenseAggregate = await Expense.aggregate([
      { $match: { ExpenseDate: { $gte: startDate, $lte: endDate } } },
      { $group: { _id: null, totalExpense: { $sum: "$Expense" } } }
    ]);
    const monthlyExpense = expenseAggregate[0]?.totalExpense || 0;
    res.locals.Exp = monthlyExpense;

    console.log(monthlyExpense);
    const Profit_month = monthlyOrders - monthlyExpense;
  } catch (err) {
    next(err);
  }
});

内容的提问来源于stack exchange，提问作者Harsh Kataria