resizeobserver loop completed with undelivered notifications.

HaC 2025

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19-03-2025

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The error "ResizeObserver loop completed with undelivered notifications" is a relatively common JavaScript issue, particularly when dealing with complex layouts or animations involving frequent DOM element resizing. This article will delve into the root causes of this error, explore effective debugging strategies, and provide practical solutions to prevent it from disrupting your web application's performance.

Understanding the ResizeObserver

The ResizeObserver API is a powerful tool for monitoring changes in the size of DOM elements. It's incredibly useful for building responsive layouts, creating smooth animations, and implementing sophisticated user interfaces. However, its efficient operation relies on careful handling of notifications. When the observer's internal loop completes, but it still has pending notifications to deliver, the error message arises.

How the Error Occurs

The error typically happens when the browser's rendering engine is overwhelmed. This can occur due to:

• Excessive Resize Events: Rapid and continuous resizing of numerous elements can flood the observer with notifications, leading to a backlog. The browser may struggle to keep up, resulting in undelivered notifications.
• Circular Dependencies: If a resize event triggers another resize event, creating a cycle, the observer might enter an infinite loop, unable to process notifications efficiently.
• Long-running Tasks in Callbacks: If the callback function associated with the ResizeObserver takes a significant amount of time to execute, subsequent resize events might be delayed or dropped, leading to undelivered notifications. The browser essentially has a queue, and a lengthy callback can block new notifications from entering the queue in time.
• Performance Bottlenecks: Other performance issues within the application (e.g., heavy computations, memory leaks) can indirectly contribute to the problem by slowing down the processing of resize events.

Debugging Strategies

Identifying the source of the error requires a systematic approach:

1. Browser Developer Tools: The browser's developer tools (specifically the console) are your first line of defense. The error message itself often provides clues about which element is causing the problem. Use the performance profiler to identify long-running tasks or bottlenecks.

2. Simplify the Layout: Temporarily simplify your layout to isolate the problematic elements. Comment out sections of code or remove elements to see if the error persists. This helps pinpoint the source of the excessive resize events.

3. Console Logging: Strategically place console.log statements within the ResizeObserver callback to track the size changes of elements and the time it takes to process them.

4. requestAnimationFrame: Consider using requestAnimationFrame within the observer's callback to throttle the rate at which notifications are processed, preventing the browser from being overwhelmed. This synchronizes updates with the browser's repaint cycle.

Solutions and Best Practices

Here's how to address the "ResizeObserver loop completed with undelivered notifications" error:

1. Debouncing and Throttling: Implement debouncing or throttling techniques to limit the frequency of callback executions. This prevents the observer from being bombarded with excessive notifications. Libraries like Lodash offer helpful utilities for this.

2. ResizeObserver Instance Management: Properly manage ResizeObserver instances. Ensure you disconnect observers when they are no longer needed using .disconnect(). This frees up resources and prevents unnecessary notification processing.

3. Optimization of Callback Functions: Optimize the callback functions associated with your ResizeObserver to minimize execution time. Avoid complex or computationally intensive operations within the callback.

4. Avoid Circular Dependencies: Carefully review your code for any circular dependencies that could trigger infinite loops. Ensure that resize events don't recursively trigger more resize events.

5. Break Down Complex Layouts: If you have a very complex layout, break it down into smaller, more manageable components. This can help improve performance and reduce the number of resize events generated.

Example of Throttling with requestAnimationFrame:

let resizeObserver = new ResizeObserver(entries => {
  const handleResize = () => {
    entries.forEach(entry => {
      // Process resize events here
      console.log("Element resized:", entry.target, entry.contentRect);
    });
  };

  requestAnimationFrame(handleResize); //Throttle the callback
});

// Observe an element
resizeObserver.observe(document.getElementById('myElement'));

Conclusion

The "ResizeObserver loop completed with undelivered notifications" error signifies a performance bottleneck related to the handling of DOM element size changes. By understanding the underlying causes and implementing the debugging strategies and best practices outlined above, you can effectively resolve this error and create robust, high-performing web applications. Remember to prioritize clean code, efficient algorithms, and proper resource management to prevent this and similar performance issues.