map函数完成后如何调用函数

Question

nodejs response returning null array. how to return a response after map function done

  var follow = [];
  User.findOne(
    { _id: id },
  ).then(user => {
    user.following.map(results => {
      User.find({ _id: results.user })
        .exec()
        .then(res => {
          follow.push(res);
     });
    });
    res.json({
      status: true,
      follow // returning null array
    });
  });
};

Answer 1

You need to collect the promises and use Promise.all() to know when they are all done:

User.findOne({ _id: id }).then(user => {
    let promises = user.following.map(results => {
      return User.find({ _id: results.user })
        .exec()
        .then(res => {
          return res;
     });
    });
    Promise.all(promises).then(follow => {
        res.json({
          status: true,
          follow // returning null array
        });
    }).catch(err => {
        console.log(err);
        res.sendStatus(500);
    });
});

Note, there is no reason in your original code to use .map() if you weren't going to return anything from the .map() callback. In my code, I return the promise.