Javascript not assigning new value to variable

Question

OK I am doing this in a node.js file. I have a function that is suppose to search for a user in mongodb, based off a certain fields. It is suppose to capture their account value and then pass it on for further evaluation. However the account variable remains null. It looks like this:

function checkIfUserExist (name, lobby, socket) {
    var account_ID = null;
    Clients.findOne({ user_name_perm: name }, function (err, result) {
        if (err) {
            console.log("Cannot find user with perm name: checkIfUserNameExist");
        } else {
            if (result == null) {
                ...} else {
                console.log("User perm name is: " + result.user_name_perm);
                console.log("result.account_ID: " + result.account_ID);
                account_ID = result.account_ID;
            }
        }
    });
    console.log("Account ID: " + account_ID);


    if (!account_ID == null) {...} else {
        console.log("User doesn't exist");
    }               
}

The log portion looks like this:

Account ID: null
User doesn't exist
User perm name is: Peter
result.account_ID: 15555555555

This is very confusing to me, it seems like the function is evaluating everything but the database look up first, which makes account_ID null for evaluation purposes, then it goes into the database and makes the new evaluation after the fact, maybe, why is it doing this?!

score 2 · Accepted Answer · answered Oct 09 '15 at 11:22

findOne runs asynchronously, so if you want to use its result you have to employ a synchronization mechanism such as passing continuations or using callbacks.

Continuation passing

function checkIfUserExist (name, lobby, socket, callback) {
    var account_ID = null;
    Clients.findOne({ user_name_perm: name }, function (err, result) {
        // snip
        console.log("Account ID:", account_ID);

        if (!account_ID == null) {
            callback(null, result);
        } else {
            console.log("User doesn't exist");
            callback(new Error);
        }  
    });                 
}

Then you have to call checkIfUserExist with a new argument -- a function that is called when the query completes and you know the result.

Promises

The mongo client you are using likely returns promises from asynchronous methods as well.

function checkIfUserExist (name, lobby, socket) {
    var account_ID = null;
    return Clients.findOne({ user_name_perm: name })
      .then(function (result) {
          console.log("Account ID:", result.account_ID);
          return result;
      }).catch(function (err) {
          // handle error
      });
}

Now you can chain .then to the return value of checkIfUserExist. You still have to pass a callback to the .then method though. May look like:

checkIfUserExist(name, lobby, socket).then(function (result) {
    response.status(200).json(result);
}).catch(function (err) {
    response.status(500).end(err.message);
});

Thanks, I figured it out, ended up altering my way of thinking when dealing with JS. Basically anywhere I wanted to assign I needed to callback, I suppose I could also just return there as well. TY — Never Nor, Oct 10 '15 at 09:18

score 0 · Answer 2 · answered Oct 09 '15 at 11:15

This is due to asynchrony

function checkIfUserExist (name, lobby, socket) {
    var account_ID = null;
    Clients.findOne({ user_name_perm: name }, function (err, result) {
        if (err) {
            console.log("Cannot find user with perm name: checkIfUserNameExist");
        } else {
            if (result == null) {
                ...} else {
                console.log("User perm name is: " + result.user_name_perm);
                console.log("result.account_ID: " + result.account_ID);
                account_ID = result.account_ID;
                console.log("Account ID: " + account_ID);

            }
        }
    });


    if (!account_ID == null) {...} else {
        console.log("User doesn't exist");
    }               
}

What is happening is that the callback in findOne is executed later than the console.log... you have to deal with asynchrony all time, I recommend https://github.com/caolan/async

You're basically right, but the code is wrong. The account_ID will be filled probably after the `if (!account_ID == null) {...} else {` is executed. You need to do everything within the callback function of the `findOne` method. — Vadorequest, Oct 09 '15 at 11:18

Javascript not assigning new value to variable

2 Answers2

Continuation passing

Promises