如何等待mongoose .exec功能完成？

Question

我是由asychronous nodejs和mongoose共同融合的.简单地说,我想发布一个用户名数组并检查,如果用户名在数据库中,那么我把它放在valid数组中,否则,把它放在invalid数组中.

这是我目前的代码:

var User = require('../../db/models/user');

api.post('/userlist', function(req, res) {

  var invalid = []; // usernames which can not be found in database
  var valid = []; // usernames which can be found in database

  (req.body.userlist).forEach(function(username) {

    User
      .findOne({username: username})
      .exec(function(err, user) {

        if (err) {
          res.send(err);
          return;

        } else if (!user) {

          invalid.push(username);

        } else {

          valid.push(req.params.item);
        }
      });
  });

  res.send({
    Invalid: invalid,
    Valid: valid
  });

});

当我执行上面的代码时,它直接输出初始空数组.

Invalid: [],
Valid: []

我知道那是因为第一次的NodeJS执行该res.send再执行功能.exec(function(err, user),但我不知道如何得到正确的invalid和valid阵列,请告知.

Answer 1

你最好的选择是使用一个承诺:

api.post('/userlist', (req, res) => {
    // Takes a username and returns a promise for information on that username.
    function findByUsername(username) {
        return new Promise((resolve, reject) =>
            User.findOne({username}).exec((err, user) => 
                err ? reject(err) : resolve(user)
            )
        );
    }

    // Iterate the array and transform each user to a promise for data on that user. 
    Promise.all(req.body.userlist.map(findByUsername))
        // Then, when all of the promises in that new array resolve
        .then(allUserDataInOrder => {
            // Find all the valid ones (if (user))
            let Valid = allUserDataInOrder.filter(Boolean); // Only those who are truthy
            // And all the invalid ones (if (!user))
            let Invalid = allUserDataInOrder.filter(userData => !userData); // Sadly, no convenient function here :(
            // And send both away
            res.send({Valid, Invalid}); // Short syntax FTW!
        })
        .catch(res.send); // Called with error object if any.
});