节点js和mongodb异步问题

Question

i'm new to mongodb and i'm having problems updating a local variable after a query. i'm using node js and i have a local variable i'm trying to update depending on my query result, but it seems that my functions returns before the query. i understand node js is asynchronous but i'm having trouble dealing with that. you can see my code below:

 function userExist(userList, username){ //var usert = new UserSchema() var exist = false UserSchema.findOne({userName: username}, function (err, usert) { if (err) return handleError(err); if (usert) { // doc may be null if no document matched exist = true } }) console.log("boolean " + bool) return exist // return username in userList // return query } 

I'm also having a different but unrelated issue where i'm trying to extract a specific value from a query result. my schema is as follow:

 //import dependency var mongoose = require('mongoose') var Schema = mongoose.Schema //create new instance of the mongoose.schema. the schema takes an //object that shows the shape of your database entries. var UserSchema = new Schema({ userName: String, userID: String, Conversations: [ { conversationID: String, messages: [ { message: String, messageID: String, sender: String, time: String } ] } ] }) //export our module to use in server.js module.exports = mongoose.model('User', UserSchema) 

i'm trying to get the values in conversations array, add a new conversation to it and push it back in the database.

An answer to either question would be really helpful and appreciated.

just for clarification this is where i'm using the userExist function:

 //Verify Username socket.on(VERIFY_USER, (nickname, callback)=>{ if(userExist(connectedUsers, nickname)){ console.log("user exist") callback({ userExist:true, user:null }) }else{ console.log("user does not exist") callback({ userExist:false, user:createUser({name:nickname, socketId:socket.id})}) } }) 

Answer 1

As already pointed out the findOne returns a promise.

You can handle the promise executing callbacks on the success or fail of the findOne result

Define two functions to pass as callbacks

function success(user){
    //no error check 
    //doc has been found
    //do something 
    } ;
function fail(err){
    console. log(err) 
    }

Then in the findOne function body

if (err) return fail(err) ;
//else
return success(user)

OR

you can wrap the userExist function body to return a promise

function userExist(userList, username){
  return new Promise(function(resolve, reject){
    var exist = false
    UserSchema.findOne({userName: username}, function (err, usert) {
      if (err) return reject(err);
      if (usert) {
        // doc may be null if no document matched
        exist = true
        resolve(exist)
      }
    })
})   
}

And when you call the userExist

 userExist(userList, username).then(function(user){
       //do something with the user
    }).catch(function(reason) {
         console.error(reason);
    });