AEM site shows “Unexpected error” due to JS Use API issues
If your AEM public site goes down intermittently with an “Unexpected error” message, it could be due to issues in JavaScript Use API components. Restart the pods to restore service, then replace deprecated code with Sling Models and improve error handling to prevent it from happening again.
Description description
Environment
۶Ƶ Experience Manager as a Cloud Service (AEMaaCS) - Sites
Issue/Symptoms
The public website experiences intermittent downtime, often displaying an error message and failing to load certain pages. Logs point to JavaScript Use API instantiation issues and HTTP 500 errors.
Symptoms include:
- Unexpected error. We are trying to bring it online. message on affected pages.
- JavaScript Use API errors like: Identifier linkDropdownLogic.js cannot be correctly instantiated by the Use API.
- Sling exceptions due to invalid references or script logic.
- Some pages fail to load while others (like the homepage) reload successfully.
Cause
The outage was caused by a JavaScript error in a custom file: Identifier _____.js cannot be correctly instantiated by the Use API.
- The code used ECMAScript destructuring:
for (const [ _, res] of items) - This syntax is incompatible with the Rhino engine used by AEM’s Sightly.
- The error occurred when the script tried to process incomplete content structures during replication.
Why It Happened on Only One Pod
Even though all publishing pods had the same code, only one failed due to:
- Incomplete Content Replication- The pod was in a temporary state with partially replicated content.
- Unsafe Iterator Handling- The JS code couldn’t handle missing or incomplete data structures.
- Resource Exhaustion- The error loop consumed system resources, leading to a crash.
- No Auto-Recovery- The pod couldn’t recover without a manual restart due to persistent resource exhaustion.
Technical Insight
- Engine Limitation: Rhino JS engine doesn’t support destructuring on Java iterators.
- Code Behavior: May work under normal conditions, but fails under stress or partial data.
- Impact: One pod entered a failure loop, while others continued running normally.
Resolution resolution
To resolve the JavaScript Use API issue:
- Restart all publishing pods to temporarily stabilize the environment. This clears the HTTP 500 errors and restores site availability.
- Next, review the JavaScript model classes causing instantiation errors. Replace deprecated JavaScript Use API components with Java-based Sling Models, and ensure all referenced classes have proper access methods.
- Update the problematic code to avoid using ECMAScript destructuring on Java iterators. Instead, use standard Java iterator patterns. Add null checks and error handling to manage incomplete content structures safely.
- Add try/catch blocks in your JavaScript Use API scripts to prevent complete failure when content is missing. Design components to degrade gracefully and avoid cascading failures.
- Finally, review component dependencies and consider using cached fallback content for critical elements like navigation. Implement circuit breakers for components that rely on external services.