Errors in web development can be frustrating, especially when they appear unexpectedly. One such issue that has been puzzling developers recently is the ResizeObserver loop completed with undelivered notifications error.
If you’ve encountered this in your console logs, you’re not alone. It’s a common warning, especially in projects involving dynamic content or responsive designs.
By the end of this article, you’ll have actionable steps to tackle the issue and ensure smoother performance in your applications.
What is “ResizeObserver Loop Completed with Undelivered Notifications”?
The error relates to the ResizeObserver API, which is a relatively modern browser API designed to help developers monitor changes to the size of DOM elements.
Unlike traditional methods of managing resizing (like `window.onresize`), ResizeObserver is better equipped for observing size changes at the element level—making it especially useful for responsive or dynamic layouts.
When this warning occurs “ResizeObserver loop completed with undelivered notifications”—it means that there are unexpected updates or notifications triggered by ResizeObserver Loop Completed with Undelivered Notifications Error that the browser couldn’t process efficiently within the current event loop cycle.
Essentially, the API is overwhelmed by frequent or recursive element updates, leading to undelivered notifications.
While this error is technically a warning (and usually doesn’t stop your code from running), it’s still important to resolve it, as it might indicate potential performance inefficiencies.
Why Does the “ResizeObserver Loop” Error Happen?
Understanding the root causes of this warning is the first step toward addressing it. Below are the common reasons why this warning may appear:
Recursive Resize Notifications
When a `ResizeObserver` makes changes to an element’s size, it can trigger additional resize events. This creates a feedback loop where ResizeObserver is repeatedly called. For instance, resizing an element might alter the layout of its child elements, which could trigger the observer to run again.
Example:
This simple CSS and observer setup demonstrates a common scenario where resizing causes recursion:
HTML structure:
“`
<div id=”container”>
<span>Resizable content</span>
</div>
“`
CSS:
“`
#container {
width: 100%;
padding-left: 10px;
}
“`
By observing the container with a `ResizeObserver` that adjusts layout sizes dynamically, you might inadvertently cause an infinite loop of DOM updates.
Heavy DOM Updates
Another frequent scenario is when an element being watched by ResizeObserver undergoes rapid and frequent updates, such as during animations or transitions. The API can struggle to manage the flood of notifications within a single event loop.
Framework-Generated Code
The warning often occurs in frameworks like React, Angular, or Vue due to their component re-renders. These frameworks update elements dynamically, making resizing notifications more prone to these warnings.
How to Fix the “ResizeObserver Loop Completed with Undelivered Notifications” Error
Now that we understand the problem, here’s how you can resolve it effectively. Follow these best practices to address the issue and improve the performance of your web app.
Debounce or Throttle the ResizeObserver
To reduce the frequency of notifications, implement debouncing or throttling logic. Both techniques aim to delay or limit how often a function is executed during rapid triggers.
Example using Debounce:
Here’s how you can debounce ResizeObserver’s callback function in JavaScript:
“`
function debounce(callback, delay) {
let timeout;
return (…args) => {
clearTimeout(timeout);
timeout = setTimeout(() => callback(…args), delay);
};
}
const resizeObserver = new ResizeObserver(
debounce((entries) => {
entries.forEach((entry) => {
console.log(entry.contentRect.width); // Process size changes
});
}, 100)
);
resizeObserver.observe(document.querySelector(“#container”));
“`
This ensures the observer waits for a set period before executing, preventing the warning from being triggered.
Disconnect ResizeObserver When Not Needed
If you no longer need to observe an element (e.g., after a user navigates away from a page), make sure to properly disconnect the observer to free resources and prevent unnecessary resize notifications.
Example:
“`
resizeObserver.unobserve(targetNode);
resizeObserver.disconnect();
“`
By disconnecting observers, you avoid stacking redundant notifications that contribute to the error.
Minimal DOM Manipulation in Callbacks
Keep changes in your `ResizeObserver` callback minimal. Avoid modifying the size or layout of observed elements directly within the callback logic, as this can trigger nested resize updates.
Inefficient Example:
“`
const observer = new ResizeObserver((entries) => {
entries.forEach((entry) => {
entry.target.style.width = entry.contentRect.width + “px”; // Avoid this!
});
});
“`
Optimized Code:
If resizing is necessary, use `requestAnimationFrame` to defer size changes until the next frame, preventing infinite loops.
“`
const observer = new ResizeObserver((entries) => {
entries.forEach((entry) => {
window.requestAnimationFrame(() => {
// Safe size adjustment
entry.target.style.width = entry.contentRect.width + “px”;
});
});
});
“`
Isolate Critical Observers
If multiple elements are being observed, this increases the complexity and likelihood of encountering this warning. Ideally, isolate and limit the elements you monitor with ResizeObserver. Observing only critical elements reduces the notification volume.
Check for Framework-Specific Workarounds
If you’re using frameworks like React, Vue, or Angular, check whether the community has reported similar issues. Often, libraries or community-driven utilities are developed to handle such cases. For instance, libraries like React Resize Detector provide optimized resize-detection hooks you can use.
Polyfill Fallbacks for Older Browsers
Occasionally, browser inconsistencies can exacerbate API issues. For older browser support, ensure any polyfills or fallbacks you’re using align with the latest specifications. For example, popular packages like resize-observer-polyfill are widely adopted.
Preventing Future ResizeObserver Issues
To future-proof your web applications, consider these additional best practices for ResizeObserver API usage:
- Test Responsiveness Thoroughly: Regularly test your layouts, especially dynamic ones, to identify points of excessive resizing.
- Monitor Logs & Fix Warnings Early: Use tools like Chrome DevTools and Lighthouse to proactively catch performance issues, including ResizeObserver warnings.
- Stay Updated: APIs evolve, and browser vendors may address current limitations in the future. Keep track of updates to the ResizeObserver specification and browser compatibility lists.
Unlocking Better Performance for Your Applications
The ResizeObserver loop completed with undelivered notifications warning serves as a helpful reminder to fine-tune how we handle dynamic DOM updates. By understanding its causes and applying best practices to manage ResizeObserver effectively, you can ensure better performance, fewer errors, and smoother user experiences.
Start implementing these strategies today, and take your web development game to the next level!
Conclusion
ResizeObserver is a powerful tool to create responsive, dynamic web pages when used correctly. While the warning “ResizeObserver loop completed with undelivered notifications” might initially seem alarming, fixing it usually involves following best practices optimizing callbacks, managing observations efficiently, and ensuring rendering remains streamlined.
By addressing this problem early, you can deliver smoother user experiences while maintaining developer sanity.
Want to explore more about JavaScript debugging and performance optimizations in web development? Keep an eye on our blog for expert insights delivered straight to you.
FAQs
What is the ResizeObserver API used for?
ResizeObserver is used to monitor element size changes and trigger updates accordingly.
Why does the ResizeObserver Loop Completed with Undelivered Notifications Errorr occour?
This error happens when resize events are not handled within the same rendering frame, leading to undelivered notifications.
How do I prevent infinite loops with ResizeObserver?
Avoid modifying observed elements inside the callback and use debouncing or requestAnimationFrame.
Can I use ResizeObserver Loop Completed with Undelivered Notifications Error with other APIs?
Yes, combining ResizeObserver with IntersectionObserver or MutationObserver can enhance performance.
