What is the purpose of the $watch, $digest, and $apply methods in AngularJS?

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Khushi Singh · Answer

The essential methods that manage data binding and model change updates in AngularJS applications are $watch, $digest, and $apply. The application’s scope tracking functions and corresponding DOM updates are supported by these AngularJS methods.

$watch

The $watch functionality enables users to observe modifications that take place in designated scope variables. Changes in the watched variable lead to the automatic execution of defined callback functions. The mechanism enables developers to run their own written code when any variable is modified. Through $watch the system notices user input modifications in scope variables which enable it to run automatic operations.

$digest

AngularJS executes the $digest method to start the digest cycle that verifies all watched variables after which the view gets updated if needed. AngularJS executes all callback functions associated with the $watch list during its cycle following detection of variable changes. User-triggered button actions, as well other user interactions cause AngularJS to call the $digest method automatically and manual invocation remains possible at anytime.

$apply

By invoking the $apply method, users can start a process known as the digest cycle manually. The $apply method serves an important purpose in manual UI update scenarios where AngularJS does not automatically track these changes. During $apply execution, AngularJS runs the $digest cycle for displaying changes in the User Interface.

$scope.$apply(function() {
 $scope.message = "Updated message";
});

The application of $apply in this case allows both scope variable message updates and the refresh of UI elements.

The changes of variables run through $watch while $digest executes the digest cycle and $apply forces updates in cases where AngularJS cannot detect them automatically.

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What is the purpose of the $watch, $digest, and $apply methods in AngularJS?

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