Issue with AngularJS for loop incorrectly incrementing when there is a single item in the array

Question

Issue with AngularJS for loop incorrectly incrementing when there is a single item in the array

Within my Angular application, there is a snippet of code that looks like this:

for (var i = 0; i < $scope.itemList.length; i++) {
    if ($scope.itemList[i].serialNumber == quickCode) {
        console.log(i)
        returnsService.getNoReceiptErrorMessages($scope.itemList[i].sku, quickCode).then(function (response) {
            console.log(i)

            }
    }
}

This piece of code contains a simple for loop with an Angular service call to the backend API inside it. The array has only one item. As a result, the value of i should always be 0. However, the console.log(i) statement before the service call prints out 0, while the one after the service call prints 1. Can anyone identify what might be causing this behavior?

javascript angularjs angular-services

Answer 1

Answer №1

When you pass the callback to the .then method, it creates a closure. This means that the for loop will continue even when the asynchronous call is made and the counter will keep increasing.

A closure execution context holds a reference to the outer scope and its variables. In this case, when the callback is executed along with the console.log, the loop has already completed and the value of the counter 'i' (accessible within the execution context) will be equal to the array size, which is 1.

To maintain the correct counter value in the callback, you can encapsulate the service call within an immediately invoked function (creating an isolated scope) and pass the current counter as an argument.

(function(count){ 
  // 'i' will now have a separate scope within this function
  returnsService.getNoReceiptErrorMessages($scope.itemList[count].sku, 
   quickCode).then(function (response) {
    console.log(count)
  }
})(i)

Answer 2

When you pass the callback to the .then method, it creates a closure. This means that the for loop will continue even when the asynchronous call is made and the counter will keep increasing.

A closure execution context holds a reference to the outer scope and its variables. In this case, when the callback is executed along with the console.log, the loop has already completed and the value of the counter 'i' (accessible within the execution context) will be equal to the array size, which is 1.

To maintain the correct counter value in the callback, you can encapsulate the service call within an immediately invoked function (creating an isolated scope) and pass the current counter as an argument.

(function(count){ 
  // 'i' will now have a separate scope within this function
  returnsService.getNoReceiptErrorMessages($scope.itemList[count].sku, 
   quickCode).then(function (response) {
    console.log(count)
  }
})(i)

Answer 3

Answer №2

The issue lies in closure, which is why the introduction of let(ReadMore) came about. By replacing var with let, you can resolve your problem. If you are not utilizing ECMA6, then wrap your call like this:

(function(variable){
  // variable scope remain intact here 
})(variable)

You can find a solution to your specific problem in this section of the above link

For further reference, check out this Fiddle: See also this Fiddle

Answer 4

The issue lies in closure, which is why the introduction of let(ReadMore) came about. By replacing var with let, you can resolve your problem. If you are not utilizing ECMA6, then wrap your call like this:

(function(variable){
  // variable scope remain intact here 
})(variable)

You can find a solution to your specific problem in this section of the above link

For further reference, check out this Fiddle: See also this Fiddle

Issue with AngularJS for loop incorrectly incrementing when there is a single item in the array

Answer №1

Answer №2

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