如何将诺言的结果返回给地图函数

Question

I have a javascript promise where I want to return the value to map function that Easier to see than explain the problem with second promise. I know I can not return value from inside of promise but i ask if there any better solution to that. Thanks

My code:

const _                 = require('lodash')
    Boom                = require('boom'),
    When                = require('when'),
    User                = require('../../models/user');

var users;
/**
 * Contact core functionality
 */
users = {


    /**
     * Find user by id
     *
     * @param       {id} user id
     * @returns     {Promise(User)} User
     */
    findById: (id) => {
        if (_.isEmpty(id)) {
            return When.reject(Boom.badRequest('User id missing.'))
        }
        return User.get(id).run().then( (user) => {
            if (user) {
                return When.resolve(user)
            }
            return When.reject(Boom.notFound('No user registered with provided id.'))
        }).error((error) => {
            return When.reject(error)
        });
    },

    /**
     * Returns all contacts of user from database
     * @returns     [{Promise(Contact)}] contacts
     */
    findContacts: (userId) => {
        if (_.isEmpty(userId)) {
            return When.reject(Boom.badRequest('User id missing.'))
        }
        return User.get(userId).run().then( (user) => {
            if (user) {
                if(_.isObject(user.contacts)){
                    user.contacts = _.map(user.contacts, (contact) => {
                        this.findById.then((user) => {
                            contact.user = user
                        })
                        return contact
                    })
                    return When.resolve(user.contacts)
                }
                return When.reject(Boom.notFound('No contacts registered with provided user.'));
            }
        }).error((error) => {
            return When.reject(error)
        })
    },
}

And the issue here:

                user.contacts = _map(user.contacts, (contact) => {
                    this.findById.then((user) => {
                        contact.user = user
                    })
                    return contact
                })

Answer 1

Since you are using the When promise library, you should use its own iterators, such as map and reduce , to process your arrays.

One thing that is incredibly important to note here though is that in When map runs "opportunistically" (runs everything at once in parallel) while reduce runs in serial with no overlaps. In other words, if you have an array of 1000 users, map will (probably) send 1000 requests to your database at once, while reduce won't finish until it has completed each database request in turn.

So your code might look like this:

// This mutates user.contacts, so you don't need to worry about the
// 'initial value' in your reducer function
When.reduce(user.contacts, (carry, contact) => {
     return this.findById.then((user) => {
         contact.user = user
     })
 });

If you do have a ton of users to process, it might make more sense to chunk your data into an array of smaller, reasonably sized arrays; you would reduce the chunks and map the users in each chunk. This would mean the array could be processed much faster than when done in serial, but without any risk of hosing your database.

edit

// This mutates user.contacts, so you don't need to worry about the
// 'initial value' in your reducer function
// It also returns the array of users as `allContacts`
var allContacts = When.reduce(user.contacts, (carry, contact) => {
     return this.findById.then((user) => {
         contact.user = user
         carry.push(user);
         return carry;
     })
 }, []);