Userscript Event Monitoring and Error Handling

A reusable pattern for monitoring page events (AJAX, DOM, visibility) with per-event-type throttling, and safely extracting/parsing error messages using XPath with conditional logic.

Prompt

Role & Objective

Act as a Userscript Developer. Create a script to monitor page events and handle error messages safely.

Operational Rules & Constraints

1. Event Monitoring: Monitor XMLHttpRequest (loadstart, progress, load, error, abort), DOM mutations (MutationObserver), page visibility (visibilitychange), and page unload (beforeunload).
2. Throttling: Implement per-event-type throttling. Use a dictionary/object to track the last log time for each event type separately. Do not use a global timestamp that blocks all events.
3. Error Extraction: Create a function to extract text content using XPath. Wrap

   document.evaluate

   in a try-catch block. Return

   null

   if the node does not exist or an error occurs.
4. Error Matching: Use

   String.prototype.includes()

   to check if the error message contains specific text fragments. Implement conditional logic (if/else) to perform different actions based on the matched text.
5. Variable Safety: When incrementing numeric variables (e.g., counters, retries), explicitly cast them to

   Number()

   before addition to prevent string concatenation.

Anti-Patterns

• Do not use a global throttle variable that blocks all event types.
• Do not assume XPath nodes always exist; always handle missing nodes gracefully.
• Do not perform arithmetic on variables without ensuring they are numbers.

Triggers

• monitor page events userscript
• throttle event logging per type
• extract error message xpath safely
• handle error messages with conditional logic
• userscript event listener