What is ng-change in Angular?

Angular is a popular front-end framework that offers a variety of directives to create interactive and dynamic web applications. Among these, the ng-change directive is crucial in managing real-time user input. In this Answer, we’ll explore what ng-change is, how it works, and look at a practical application in Angular.

Understanding ng-change

ng-change is an AngularJS directive used to specify custom behavior when the value of an input element changes. It allows developers to execute an expression or a function in response to user interactions that modify the value of input elements like text fields, checkboxes, radio buttons, and select boxes. It is often combined with the ng-model directive to bind the model value to the input and trigger specific functions or logic in response to user input.

While both ng-change and ng-model are closely related, they serve different purposes:

• ng-model: It binds an input element’s value to a property on the scope.

• ng-change: It triggers an expression or function when the input value changes, offering a way to react to changes without directly binding to the scope.

How ng-change works?

When you use ng-change on an input element, AngularJS watches for changes in the model associated with that input via ng-model. Whenever the value changes (e.g., the user types in a text box or selects a different option), the expression or function specified in ng-change is executed.

The basic syntax for ng-change is:

It takes a function or an action as a parameter and calls it when the value of a particular element changes. In the above syntax validateUsername is called whenever the input value changes.

How to use ng-change in forms

ng-change is useful in form validations, dynamic form behaviors, and providing real-time feedback to users. Let’s look at how we can integrate ng-change within some different form elements into a form.

For a textbox

In this scenario, every time the user modifies the username field, the checkUsernameAvailability function is triggered to verify if the username is available.

For a checkbox

Here, when the checkbox state changes (checked or unchecked), the toggleSubscription function is executed to handle the subscription logic.

For a drop-down

When the selected country changes, updateStates can be called to populate the state/province options accordingly.

Code example of ng-change

Now that we have looked at how to use ng-change in form elements, let’s look at a practical example where we will do some real-time data validation. We will create a simple input field and log the number of times a change was detected, using ng-change, in the field.

ng-change is a useful directive in AngularJS that allows developers to execute custom logic in response to changes in form input values. Whether it’s for real-time validations, dynamic UI updates, or handling user interactions, ng-change provides a straightforward way to enhance the responsiveness and interactivity of your applications.