停止执行其他代码

Question

I have a request:

  Character.count({'character.ownerid': msg.author.id}, function (err, count) {
    if (err) {
      throw err;
    }

    if (count > 3) {
      err.message = 'Too many characters';
      //create error explanation and throw it
      throw err;
    }
  })

If any of there errors do happen I need to exit the whole parent function. I can't put a return in this request since it exits only this method. I thought there is a possibility to do a callback like:

Character.count({'character.ownerid': msg.author.id}, function (err, count, stop) {

But how to work with it? It is inside an anonymous function, I don't know where to place its contents. I also tried to use try / catch but I can't throw errors to the outside handler because of Error: Unhandled "error" event. ([object Object]) Error: Unhandled "error" event. ([object Object]) , see code below:

Character.count({'character.ownerid': msg.author.id}, function (err, count) {
  if (err) {
    throw err;
  }

  if (count > 3) {
    var err = {};
    err.message = 'Too many characters';
    throw err;
  }
}).then((count) => {
  console.log('all good we may continue');
  console.log(count);
}).catch((err) => {
  if (err != undefined && err.length > 0) {
    msg.reply(err.message);
    return 0; //exit parent function?
  }
});

But even if this worked I am not sure if this code will do what I need. The request is asynchronous so what if the rest of the code gets triggered before then ? Is that even possible?

So I basically need to return 0; the parent function somehow and if there are any errors I need to have a handler for them. Any ideas for this?

Answer 1

You seem to be missing the concept. Firstly as already stated, all mongoose operations return a Promise or at least a "Promise like" object which can immediately be resolved via a then() instead of passing in a callback function. This can either present in two ways.

Either with async/await syntax and a try..catch block:

const { Schema } = mongoose = require('mongoose');

const uri = 'mongodb://localhost/test';

mongoose.set('debug', true);
mongoose.Promise = global.Promise;

const characterSchema = new Schema({
  name: String
});

const Character = mongoose.model('Character', characterSchema);

const log = data => console.log(JSON.stringify(data,undefined,2));

const doCount = async () => {
  let count = await Character.count();

  if (count > 3)
    throw new Error("too many charaters");

  return count;

};


(async function() {

  try {
    const conn = await mongoose.connect(uri);

    await Promise.all(Object.entries(conn.models).map(([k,m]) => m.remove()))

    await Character.insertMany(
      ["Huey","Duey","Louie"].map(name => ({ name }))
    );

    let count = await doCount();
    log({ count });

    await Character.create({ name: 'Donald' });

    let newCount = await doCount();
    console.log("never get here");


  } catch(e) {
    console.error(e)
  } finally {
    mongoose.disconnect();
  }

})()

Or with standard then() and catch() syntax:

const { Schema } = mongoose = require('mongoose');

const uri = 'mongodb://localhost/test';

mongoose.set('debug', true);
mongoose.Promise = global.Promise;

const characterSchema = new Schema({
  name: String
});

const Character = mongoose.model('Character', characterSchema);

const log = data => console.log(JSON.stringify(data,undefined,2));

function doCount() {
  return Character.count()
    .then(count =>  {

      if (count > 3)
        throw new Error("too many charaters");

      return count;
    });

};


(function() {

  mongoose.connect(uri)
    .then(conn => Promise.all(
      Object.entries(conn.models).map(([k,m]) => m.remove())
    ))
    .then(() => Character.insertMany(
      ["Huey","Duey","Louie"].map(name => ({ name }))
    ))
    .then(() => doCount())
    .then(count => log({ count }))
    .then(() => Character.create({ name: 'Donald' }))
    .then(() => doCount())
    .then(() => console.log("never get here"))
    .catch(e => console.error(e))
    .then(() => mongoose.disconnect() );

})()

And the output of both listings is just the same:

Mongoose: characters.remove({}, {})
Mongoose: characters.insertMany([ { _id: 5b0f66ec5580010efc5d0602, name: 'Huey', __v: 0 }, { _id: 5b0f66ec5580010efc5d0603, name: 'Duey', __v: 0 }, { _id: 5b0f66ec5580010efc5d0604, name: 'Louie', __v: 0 } ], {})
Mongoose: characters.count({}, {})
{
  "count": 3
}
Mongoose: characters.insertOne({ _id: ObjectId("5b0f66ec5580010efc5d0605"), name: 'Donald', __v: 0 })
Mongoose: characters.count({}, {})
Error: too many charaters
    at doCount (/home/projects/characters/index.js:20:11)
    at <anonymous>
    at process._tickCallback (internal/process/next_tick.js:188:7)

As you can see the function happily returns it's value where the count remains 3 or under, but then throws an exception which stops further execution when the count would be greater than 3 since the "never get here" message never gets logged.

So there is no need for "callbacks" here and you would not use one unless you wrapped it in a Promise can did the same type of error handling anyway.

But if you have an "error" then throw the error. This works fine in a promise chain, but a "callback" which does not return as a Promise is simply not part of that chain and can never be "caught". So simply don't use the callback when you don't need to.

Just for kicks, wrapping a callback with a Promise would be done like:

function doCount() {
  return new Promise((resolve,reject) =>
    Character.count().exec((err,count) => {

      if (count > 3)
        reject(new Error("too many charaters"));

      resolve(count);
    })
  );
};

But it's noted not to be necessary considering the native methods return something you can resolve as a Promise anyway.