Preventing $digest cycle from triggering on every event

In AngularJS, the $digest cycle is responsible for detecting changes and updating the DOM accordingly. However, frequent triggering of the $digest cycle (especially on high-frequency events like mousemove, scroll, and keypress) can severely impact performance.

This guide covers:

Understanding the $digest cycle
Why frequent $digest cycles are a problem
Methods to prevent unnecessary $digest triggers
Best practices for optimizing AngularJS performance

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1. Understanding the $digest Cycle

AngularJS uses a two-way data binding system, where changes in the model automatically update the view and vice versa. This synchronization happens through a digest cycle, which:

• Scans all $scope variables
• Checks for changes
• Updates the DOM if needed

Whenever AngularJS detects a change (e.g., button click, API response, event listener), it triggers a new $digest cycle.

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2. Why Frequent $digest Cycles Cause Performance Issues

Certain events, like mousemove and scroll, fire continuously while the user interacts with the page. If AngularJS triggers a $digest cycle every time, the app can experience:

• Laggy UI
• Unnecessary re-renders
• Increased CPU usage
• Slower performance on low-end devices

Thus, we need to prevent unnecessary digest cycles while still handling events efficiently.

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3. Ways to Prevent Unnecessary $digest Cycles

A. Use $scope.$applyAsync() Instead of $apply()

Normally, event handlers use $scope.$apply(), which immediately triggers a digest cycle. Instead, using $scope.$applyAsync() delays execution until the current cycle is complete.

Example: Using $applyAsync() to batch updates

app.controller("MainCtrl", function($scope) {
    document.addEventListener("mousemove", function(event) {
        $scope.$applyAsync(function() {
            $scope.mouseX = event.clientX;
            $scope.mouseY = event.clientY;
        });
    });
});

Why?

• Prevents multiple $digest cycles from running unnecessarily
• Groups multiple updates into one efficient cycle

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B. Use $timeout Instead of $apply() for Asynchronous Tasks

If an event updates the UI frequently, wrapping it in $timeout avoids forcing immediate digest cycles.

Example: Using $timeout to reduce digest cycles

app.controller("MainCtrl", function($scope, $timeout) {
    document.addEventListener("scroll", function() {
        $timeout(function() {
            $scope.scrollPosition = window.scrollY;
        }, 200);  // Updates at most every 200ms
    });
});

Why?

• The $digest cycle only runs after 200ms, reducing strain on the system
• The UI updates at a controlled frequency

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C. Use $evalAsync() for Deferred Execution

If you want to schedule a digest cycle but allow other tasks to finish first, $evalAsync() is useful.

Example: Deferring Execution with $evalAsync()

app.controller("MainCtrl", function($scope) {
    document.addEventListener("keydown", function(event) {
        $scope.$evalAsync(function() {
            $scope.lastKeyPressed = event.key;
        });
    });
});

Why?

• Prevents immediate $digest cycles for every keystroke
• Ensures updates don’t block other tasks

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D. Bind Heavy Event Listeners Outside Angular’s Digest Scope

If an event doesn’t need to update Angular’s data binding, use $scope.$apply() only when necessary.

Example: Using window.addEventListener Without Triggering $digest

app.controller("MainCtrl", function($scope) {
    document.addEventListener("mousemove", function(event) {
        console.log("Mouse moved:", event.clientX, event.clientY);
        // No need for $apply since this doesn't update Angular scope
    });
});

Why?

• Prevents unnecessary digest cycles for non-Angular logic
• Reduces CPU-intensive tasks

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E. Use $scope.$on('$destroy') to Clean Up Listeners

When controllers or directives are destroyed, clean up event listeners to prevent memory leaks.

Example: Cleaning Up Listeners on Scope Destroy

app.controller("MainCtrl", function($scope) {
    function handleScroll() {
        console.log("Scrolling:", window.scrollY);
    }

    window.addEventListener("scroll", handleScroll);

    $scope.$on("$destroy", function() {
        window.removeEventListener("scroll", handleScroll);
    });
});

Why?

• Prevents unused event listeners from keeping memory occupied
• Improves performance in single-page applications (SPA)

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F. Use track by in ng-repeat to Reduce Digest Overhead

When looping through large datasets, AngularJS creates a new $digest cycle for every item. Use track by to reduce unnecessary cycles.

Example: Optimizing ng-repeat with track by

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

Why?

• Prevents Angular from re-checking unchanged list items
• Greatly improves performance in large lists

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4. Best Practices for Reducing $digest Cycle Execution Time

✔️ Use $scope.$applyAsync() instead of $apply()
✔️ Use $timeout() to throttle high-frequency updates
✔️ Bind event listeners outside of AngularJS when possible
✔️ Destroy event listeners using $scope.$on('$destroy')
✔️ Optimize ng-repeat with track by
✔️ Use $evalAsync() for deferred execution
✔️ Avoid deep watching large objects ($watchCollection() instead of $watch())