Excessive digest cycles causing UI lag

In AngularJS, the digest cycle is responsible for updating the UI when model data changes. However, if too many digest cycles are triggered, it can cause severe UI lag, slow performance, and unresponsive pages.

This guide will break down why excessive digest cycles happen and how to optimize AngularJS performance effectively.

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1. Understanding the AngularJS Digest Cycle

How the Digest Cycle Works?

• When the model ($scope data) changes, AngularJS starts a digest cycle.
• It checks all watchers ($watch expressions) to detect changes.
• If a change is found, it runs another cycle to check for further updates.
• This process repeats until no further changes are detected (max: 10 iterations).

Example of How AngularJS Triggers a Digest Cycle

$scope.name = "John"; // A digest cycle is triggered automatically

Any Angular directive, like ng-model, ng-click, or ng-repeat, can also trigger digest cycles.

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2. Symptoms of Excessive Digest Cycles

If your AngularJS app is slow, check for these common symptoms:

Laggy UI: UI updates feel slow, especially on user input events.
High CPU Usage: The browser’s CPU usage spikes, even when idle.
Slow Animations: CSS transitions and JavaScript animations stutter.
Console Errors: You see 10 digest iterations reached errors.
Too Many Watchers: Running the following script in the browser console shows thousands of watchers:

(function () {
    var totalWatchers = 0;
    var countWatchers = function (element) {
        if (angular.element(element).scope()) {
            totalWatchers += angular.element(element).scope().$$watchers ? angular.element(element).scope().$$watchers.length : 0;
        }
        angular.forEach(element.children, function (child) {
            countWatchers(child);
        });
    };
    countWatchers(document.body);
    console.log("Total Watchers: ", totalWatchers);
})();

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3. Common Causes of Excessive Digest Cycles & Solutions

Cause 1: Too Many Watchers ($watch, $watchCollection)

Every ng-model, ng-repeat, and $watch adds watchers. More watchers mean longer digest cycles.

Bad Example (Too Many Watchers)

$scope.$watch('user.name', function (newVal, oldVal) {
    console.log("User changed:", newVal);
});

Each $watch adds overhead to the digest cycle.

Solution: Use One-Time Binding (::)

<p>{{ ::user.name }}</p>

Why? Removes the watcher after initial binding, reducing digest load.

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Cause 2: Unnecessary $apply() Calls

Calling $scope.$apply() inside event listeners triggers digest cycles unnecessarily.

Bad Example ($apply() inside setTimeout)

setTimeout(function () {
    $scope.$apply(function () {
        $scope.name = "John";
    });
}, 1000);

Fix: Use $timeout Instead (Optimized for AngularJS)

$timeout(function () {
    $scope.name = "John";
});

Why? $timeout automatically triggers digest cycles efficiently.

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Cause 3: Using $digest() Inside a Loop

Calling $scope.$digest() inside a loop triggers multiple digest cycles, slowing down performance.

Bad Example (Multiple $digest() Calls)

for (var i = 0; i < 100; i++) {
    $scope.$digest(); // Triggers 100 digest cycles!
}

Fix: Batch Updates and Use $evalAsync()

$scope.$evalAsync(function () {
    for (var i = 0; i < 100; i++) {
        $scope.list.push(i);
    }
});

Why? $evalAsync() schedules a digest cycle only once after the loop.

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Cause 4: Heavy ng-repeat with Large Lists

Using ng-repeat with thousands of items creates excessive watchers, leading to slow digest cycles.

Bad Example (Rendering Large List)

<div ng-repeat="item in items">
    {{ item.name }}
</div>

Fix: Use track by, Pagination, or Virtual Scroll

<div ng-repeat="item in items track by item.id">
    {{ item.name }}
</div>

Why? Prevents Angular from recreating DOM elements unnecessarily.

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Cause 5: Filters Inside ng-repeat

Applying filters directly inside ng-repeat triggers digest cycles for every item.

Bad Example (Filter in ng-repeat)

<div ng-repeat="user in users | orderBy:'name'">
    {{ user.name }}
</div>

Fix: Apply Filters in the Controller

$scope.sortedUsers = $filter('orderBy')($scope.users, 'name');

<div ng-repeat="user in sortedUsers">
    {{ user.name }}
</div>

Why? Filters run once, reducing digest cycles.

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Cause 6: Too Many ng-if Directives

Using ng-if instead of ng-show removes and recreates DOM elements, triggering extra digest cycles.

Bad Example (ng-if Overuse)

<div ng-if="isVisible">Visible</div>

Fix: Use ng-show Instead

<div ng-show="isVisible">Visible</div>

Why? ng-show just toggles display: none, avoiding DOM recreation.

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4. Best Practices to Optimize Digest Cycles

Issue	Solution
Too many watchers slowing down digest cycles	Use one-time binding (::), minimize $watch usage
Unnecessary $apply() calls triggering multiple cycles	Use $timeout or $evalAsync() instead
Loops triggering $digest() multiple times	Batch updates and use $evalAsync()
Large lists causing slow digest cycles	Use pagination, virtual scroll, track by
Filters inside ng-repeat triggering excessive checks	Move filters to the controller
ng-if creating unnecessary DOM changes	Use ng-show instead