JavaScript failing to update variable with new value

Question

JavaScript failing to update variable with new value

Alright, so here's the situation. I'm working on a node.js file where I've got a function set up to search for a user in mongodb using specific fields. The goal is to retrieve their account value and then pass it along for further processing. But for some reason, the account variable keeps turning out null. Here's a snippet of what I have:

function checkIfUserExist (name, lobby, socket) {
    var account_ID = null;
    Clients.findOne({ user_name_perm: name }, function (err, result) {
        if (err) {
            console.log("Can't 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");
    }               
}

And when I log everything, this is what shows up:

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

This whole process is really puzzling me. It seems like the function is going through all the steps except for the database lookup first, which ends up leaving account_ID as null initially for evaluation purposes. Then after that, it jumps into the database and does the evaluation later. This just doesn't make sense to me, why is it behaving this way?!

javascript mongodb

Answer 1

Answer №1

findOne operates asynchronously, meaning that in order to utilize its outcome, you must incorporate a synchronization technique like utilizing continuations or employing callbacks.

Using Continuations

function verifyUserExistence (name, lobby, socket, callback) {
    var accountID = null;
    Clients.findOne({ user_name_perm: name }, function (err, result) {
        // code snippet
        console.log("Account ID:", accountID);

        if (!accountID == null) {
            callback(null, result);
        } else {
            console.log("User does not exist");
            callback(new Error);
        }  
    });                 
}

In this case, you need to invoke verifyUserExistence with an additional argument - a function that is executed once the query is finished and the result is known.

Using Promises

The MongoDB client being utilized likely also returns promises from asynchronous methods.

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

You are now able to chain .then to the return value of verifyUserExistence. However, you still need to supply a callback to the .then method. It could resemble the following:

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

Answer 2

findOne operates asynchronously, meaning that in order to utilize its outcome, you must incorporate a synchronization technique like utilizing continuations or employing callbacks.

Using Continuations

function verifyUserExistence (name, lobby, socket, callback) {
    var accountID = null;
    Clients.findOne({ user_name_perm: name }, function (err, result) {
        // code snippet
        console.log("Account ID:", accountID);

        if (!accountID == null) {
            callback(null, result);
        } else {
            console.log("User does not exist");
            callback(new Error);
        }  
    });                 
}

In this case, you need to invoke verifyUserExistence with an additional argument - a function that is executed once the query is finished and the result is known.

Using Promises

The MongoDB client being utilized likely also returns promises from asynchronous methods.

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

You are now able to chain .then to the return value of verifyUserExistence. However, you still need to supply a callback to the .then method. It could resemble the following:

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

Answer 3

Answer №2

The reason for this issue lies in the concept of 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");
    }               
}

The issue arises because the callback in findOne is executed after the console log statements. Asynchronous behavior must be dealt with consistently, and I suggest using https://github.com/caolan/async to handle such situations.

Answer 4

The reason for this issue lies in the concept of 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");
    }               
}

The issue arises because the callback in findOne is executed after the console log statements. Asynchronous behavior must be dealt with consistently, and I suggest using https://github.com/caolan/async to handle such situations.

JavaScript failing to update variable with new value

Answer №1

Using Continuations

Using Promises

Answer №2

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