Undefined Javascript variables are commonly found within loops

Question

Undefined Javascript variables are commonly found within loops

My code includes a loop that fetches an array of users and attempts to retrieve the information of each user. However, I've encountered an issue where if there are multiple users, it ends up returning the same information (specifically, the details of the last user) twice. Consequently, when I attempt to update and save the user information, only the data of the last user gets saved.

for (i = 0; i < users.length; i++) { 
    var amount = users[i].monthlyAmount;

    if(amount != '0') {
      var stripeID = users[i].stripeID;
      accountID =  users[i];
      
      stripe.charges.create({
        amount: amount,
        currency: "usd",
        customer: stripeID, 
        description: "Monthly charge"
        }, function(err, charge) {
        if (err) return (err);

        if(!err) {
          console.log(accountID);
          accountID.monthlyAmount = '0';
          accountID.save();
        }
      });
    }
  }

Below is the current output:

{ billingInfo: 'true',
createdAt: Sun Nov 16 2014 14:05:21 GMT-0600 (CST),
email: '<a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="85e4c5e4abe6eae8">[email protected]</a>',
encryptedPassword: '*removed',
monthlyAmount: 1000,
propertyCount: 0,
stripeID: '*removed',
updatedAt: Sun Nov 16 2014 15:10:59 GMT-0600 (CST),
id: '54690381c03a265b07c99564' }
{ billingInfo: 'true',
createdAt: Sun Nov 16 2014 14:05:21 GMT-0600 (CST),
email: '<a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="2a4b6a4b04494547">[email protected]</a>',
encryptedPassword: '*removed',
monthlyAmount: '0',
propertyCount: 0,
stripeID: '*removed',
updatedAt: Sun Nov 16 2014 15:10:59 GMT-0600 (CST),
id: '54690381c03a265b07c99564' }

javascript arrays json variables sails.js

Answer 1

Answer №1

To ensure the correct scope of the iterating variable "i", it is important to create a closure. By doing so, you can prevent issues that may arise from the value of "i" being overwritten in a loop. Think of a closure as a safeguarded environment for your values. Refer to Aravind's post for further clarification.

for (i = 0; i < users.length; i++) {
  (function(i){
    var amount = users[i].monthlyAmount;
    
    if(amount != '0') {
      var stripeID = users[i].stripeID;
      accountID =  users[i];
      
      stripe.charges.create({
        amount: amount,
        currency: "usd",
        customer: stripeID, 
        description: "Monthly charge"
        }, function(err, charge) {
        if (err) return (err);
        
        if(!err) {
          console.log(accountID);
          accountID.monthlyAmount = '0';
          accountID.save();
        }
      });
    }
  }(i))
  }

Answer 2

To ensure the correct scope of the iterating variable "i", it is important to create a closure. By doing so, you can prevent issues that may arise from the value of "i" being overwritten in a loop. Think of a closure as a safeguarded environment for your values. Refer to Aravind's post for further clarification.

for (i = 0; i < users.length; i++) {
  (function(i){
    var amount = users[i].monthlyAmount;
    
    if(amount != '0') {
      var stripeID = users[i].stripeID;
      accountID =  users[i];
      
      stripe.charges.create({
        amount: amount,
        currency: "usd",
        customer: stripeID, 
        description: "Monthly charge"
        }, function(err, charge) {
        if (err) return (err);
        
        if(!err) {
          console.log(accountID);
          accountID.monthlyAmount = '0';
          accountID.save();
        }
      });
    }
  }(i))
  }

Answer 3

Answer №2

The reason for this issue is due to the asynchronous nature of function calls, which can be explained as follows.

for (i = 0; i < users.length; i++) { 
      stripe.charges.create({
           ...
        }, function(err, charge) {
            //callback function executed after stripe.charges.create completes
        });
}

i is initialized to 0
stripe is called for users[0].stripeID, and while the execution is in progress, it moves on to step (3).
i is incremented to 1
stripe is called for users[1].stripeID, and similarly, without waiting for completion, proceeds to step (5).
Execution of the for loop is completed at this stage.
Upon completion of step (2), the callback function is triggered. It references accountID, which is the current accountID set recently in step (4).
Finally, step (4) finishes, triggering its callback and displaying the most recent accountID obtained in that step.

To address this issue:

for (i = 0; i < users.length; i++) { 
    var amount = users[i].monthlyAmount;
    if(amount != '0') {
      var stripeID = users[i].stripeID;
      var accountID =  users[i];
      function callback(){
        var account = accountID;
        return function(err, charge) {
        if (err) return (err);
        if(!err) {
          console.log(accountID);
          accountID.monthlyAmount = '0';
          accountID.save();
        };
      }
      
      stripe.charges.create({
        amount: amount,
        currency: "usd",
        customer: stripeID, 
        description: "Monthly charge"
        }, callback()
      });
    }
  }

Answer 4

The reason for this issue is due to the asynchronous nature of function calls, which can be explained as follows.

for (i = 0; i < users.length; i++) { 
      stripe.charges.create({
           ...
        }, function(err, charge) {
            //callback function executed after stripe.charges.create completes
        });
}

i is initialized to 0
stripe is called for users[0].stripeID, and while the execution is in progress, it moves on to step (3).
i is incremented to 1
stripe is called for users[1].stripeID, and similarly, without waiting for completion, proceeds to step (5).
Execution of the for loop is completed at this stage.
Upon completion of step (2), the callback function is triggered. It references accountID, which is the current accountID set recently in step (4).
Finally, step (4) finishes, triggering its callback and displaying the most recent accountID obtained in that step.

To address this issue:

for (i = 0; i < users.length; i++) { 
    var amount = users[i].monthlyAmount;
    if(amount != '0') {
      var stripeID = users[i].stripeID;
      var accountID =  users[i];
      function callback(){
        var account = accountID;
        return function(err, charge) {
        if (err) return (err);
        if(!err) {
          console.log(accountID);
          accountID.monthlyAmount = '0';
          accountID.save();
        };
      }
      
      stripe.charges.create({
        amount: amount,
        currency: "usd",
        customer: stripeID, 
        description: "Monthly charge"
        }, callback()
      });
    }
  }

Undefined Javascript variables are commonly found within loops

Answer №1

Answer №2

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