graphql-yoga and CRUD example with subscription

resolver.js

 Subscription: {
        user: {
            subscribe: (parent, args, {pubSub}) => {
                return pubSub.asyncIterator([MutationTypes.created, MutationTypes.updated])
            }
        }
    }

Mutation: {
        updateUser: async (parent, {_id, email, firstName, lastName}, {collections, ObjectID, pubSub}) => {
            const form = getUserObj({email, firstName, lastName})
            try {
                delete form._id // need to remove the _id modifier
                const res = await collections.users.updateOne(
                    {_id: ObjectID(_id)},
                    {$set: form}
                )
                if (res.matchedCount !== 1) {
                    throw new Error('error.user_not_found')
                }
                const user = await collections.users.findOne({_id: ObjectID(_id)})
                Object.assign(user, {
                    _id: _id
                })
                pubSub.publish(MutationTypes.updated, {
                 user:{
                    mutationType: MutationTypes.updated,
                    node: user
                 }
                })
                return res
            } catch (e) {
                console.log(e)
                throw new Error(e.message)
            }

        },

schema.gql

type SubscriptionPayload{
    mutationType:String
    node:User
}

type Subscription{
    user:SubscriptionPayload!
}