What is the $digest cycle in AngularJS?

In AngularJS, the $digest cycle is the mechanism responsible for detecting changes in the application and updating the UI accordingly. It is part of AngularJS’s two-way data binding, ensuring that changes in the Model (JavaScript objects) reflect in the View (HTML) and vice versa.

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How Does the $digest Cycle Work?

1. Triggered Automatically – When an AngularJS event (e.g., ng-click, ng-model update, $http response) occurs, the $digest cycle starts.
2. Watches for Changes – It checks all watch expressions ($scope.$watch) to see if any data has changed.
3. Propagates Changes – If changes are found, Angular updates the View and triggers another check to ensure everything is updated correctly.
4. Stops When Stable – The cycle repeats until no more changes are detected (usually 2-10 iterations).

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Example of $digest Cycle in Action

Let’s consider a simple example:

<div ng-app="myApp" ng-controller="MainController">
    <p>Counter: {{ counter }}</p>
    <button ng-click="increment()">Increment</button>
</div>

var app = angular.module("myApp", []);

app.controller("MainController", function ($scope) {
    $scope.counter = 0;

    $scope.increment = function () {
        $scope.counter++; // Changes the Model
    };
});

🔹 What Happens Internally?

• When the user clicks the button, increment() updates $scope.counter.
• This triggers the $digest cycle, which checks all $scope variables.
• Since $scope.counter has changed, the View updates automatically.

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$digest vs $apply

1️⃣ $digest()

• Runs the digest cycle only on the current $scope and its children.
• Used when you want to manually trigger a limited update.
• Example: javascriptCopyEdit$scope.$digest();

2️⃣ $apply()

• Runs the digest cycle on the entire application ($rootScope and all child scopes).
• Used when changes happen outside of AngularJS (e.g., inside setTimeout, jQuery).
• Example: javascriptCopyEdit$scope.$apply(function () { $scope.counter++; }); This ensures Angular is aware of the change.

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Common Pitfalls with $digest Cycle

1️⃣ $digest already in progress Error

If you manually call $digest() inside an AngularJS event (e.g., ng-click), it conflicts with the existing cycle. Instead, use $apply():

Correct:

$scope.$apply(function () {
    $scope.counter++;
});

Incorrect:

$scope.increment = function () {
    $scope.counter++;
    $scope.$digest(); //  Error: $digest already in progress
};

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2️⃣ Performance Issues with Large Applications

• Too many watchers ($scope.$watch) slow down performance.
• Avoid deep object watching (true flag in $watch).
• Use one-time binding (::) when data does not change: <p>Name: {{ ::user.name }}</p>