What is the significance of implementing a function in AngularJS that calculates the sum of two numbers?

After utilizing angularjs in various applications for some time, I find myself perplexed by the seemingly straightforward yet elusive nature of Angular.

Here is the code snippet causing my confusion:

<!DOCTYPE html>
<html>
<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.6.4/angular.min.js"></script>
<body>

<div ng-app="myApp" ng-controller="myCtrl">

First Number: <input type="number" ng-model="firstNumber"><br>
Last Number: <input type="number" ng-model="secondNumber"><br>
<br>
Sum: {{result()}}

</div>

<script>
var app = angular.module('myApp', []);
app.controller('myCtrl', function($scope) {
    $scope.firstNumber = 0 ;
    $scope.secondNumber = 0;
    $scope.result = function() {
        return $scope.firstNumber + $scope.secondNumber;
    };
});
</script>

</body>
</html>

The above code functions correctly. However, my challenge lies in wanting the following code to operate as expected:

<script>
    var app = angular.module('myApp', []);
    app.controller('myCtrl', function($scope) {
        $scope.firstNumber = 0 ;
        $scope.secondNumber = 0;
        $scope.result = $scope.firstNumber + $scope.secondNumber;

    });
    </script>

and within the view:

Sum : {{result}}

Despite being encapsulated within the scope context, why does the sum value not update in the view upon user input changes?

I seek assistance understanding this scenario!

Answer №1

When the model is updated, the displayed result will change accordingly due to continuous updates in a specific function. Initially, all JavaScript statements are executed only once during initialization of the controller, resulting in the evaluation of the value and view. However, as values are modified, Angular automatically updates the model for firstNumber and secondNumber using input field values—without additional directives prompting further action upon their modification. Consequently, neither JavaScript nor Angular re-evaluates all controller-defined JavaScript statements, as this behavior is inherent to the controller's definition.

The crux of the matter then becomes what transpires when the expression {{ firstNumber + secondNumber }} is utilized. In this scenario, Angular incessantly evaluates the values of these models in conjunction with their respective views—notwithstanding the absence of explicit instructions targeting the JavaScript statement within the controller's definition responsible for summing said numbers.

Answer №2

The reason for this discrepancy is due to the differing data model of your outcome, eliminating the necessity for a function. To showcase the sum, utilize the following code:

First Number: <input type="number" ng-model="firstNumber"><br>
Last Number: <input type="number" ng-model="secondNumber"><br>
<br>
Sum: {{firstNumber + secondNumber}}

Additonally, {{result()}} will automatically execute each time there is a modification in the data model. For precise calculation of the sum solely upon changes to firstNumber or secondNumber, consider implementing $scope.$watch or ng-change.

Answer №3

AngularJS controllers fire each function only once.

The first tip is to use a watch in the function:

$scope.result = function(){   
      return $scope.firstNumber + $scope.secondNumber;
   $scope.$watchGroup(['firstNumber ', 'secondNumber'], 
    function(newValues, oldValues, scope) {
    return newValues[0]+newValues[1]
    });
}

The second tip is to use ng-change to execute for every change:

First Number: <input type="number" ng-change="result()" ng-model="firstNumber"><br>
Last Number: <input type="number" ng-change="result()" ng-model="secondNumber"><br>

In the controller:

$scope.result = function() {
        return $scope.firstNumber + $scope.secondNumber;
    };

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