What are o.js Error Messages?

General o.js Runtime Errors

o.js file errors often occur during the startup phase of Calibre, but can also occur while the program is running. These types JS errors are also known as “runtime errors” because they occur while Calibre is running. Here are some of the most common o.js runtime errors:

• o.js could not be found.
• o.js error.
• o.js failed to load.
• Error loading o.js.
• Failed to register o.js / Cannot register o.js.
• Runtime Error - o.js.
• The file o.js is missing or corrupt.

Most JS errors are due to missing or corrupt files. Your o.js file could be missing due to accidental deletion, uninstalled as a shared file of another program (shared with Calibre), or deleted by a malware infection. Furthermore, o.js file corruption could be caused from a power outage when loading Calibre, system crash while loading or saving o.js, bad sectors on your storage media (usually your primary hard drive), or malware infection. Thus, it’s critical to make sure your anti-virus is kept up-to-date and scanning regularly.

How to Fix o.js Errors in 3 Steps (Time to complete: ~5-15 minutes)

If you're encountering one of the error messages above, follow these troubleshooting steps to resolve your o.js issue. These troubleshooting steps are listed in the recommended order of execution.

To begin System Restore (Windows XP, Vista, 7, 8, and 10):

1. Hit the Windows Start button
2. When you see the search box, type "System Restore" and press "ENTER".
3. In the search results, find and click System Restore.
4. Please enter the administrator password (if applicable / prompted).
5. Follow the steps in the System Restore Wizard to choose a relevant restore point.
6. Restore your computer to that backup image.

If the Step 1 fails to resolve the o.js error, please proceed to the Step 2 below.

You can uninstall Calibre software by following these instructions (Windows XP, Vista, 7, 8, and 10):

1. Hit the Windows Start button
2. In the search box, type "Uninstall" and press "ENTER".
3. In the search results, find and click "Add or Remove Programs"
4. Find the entry for Calibre 4.2.0 and click "Uninstall"
5. Follow the prompts for uninstallation.

After the software has been fully uninstalled, restart your PC and reinstall Calibre software.

If this Step 2 fails as well, please proceed to the Step 3 below.

Calibre 4.2.0

Kovid Goyal

When the first two steps haven't solved your issue, it might be a good idea to run Windows Update. Many o.js error messages that are encountered can be contributed to an outdated Windows Operating System. To run Windows Update, please follow these easy steps:

1. Hit the Windows Start button
2. In the search box, type "Update" and press "ENTER".
3. In the Windows Update dialog box, click "Check for Updates" (or similar button depending on your Windows version)
4. If updates are available for download, click "Install Updates".
5. After the update is completed, restart your PC.

If Windows Update failed to resolve the o.js error message, please proceed to next step. Please note that this final step is recommended for advanced PC users only.

If Those Steps Fail: Download and Replace Your o.js File (Caution: Advanced)

If none of the previous three troubleshooting steps have resolved your issue, you can try a more aggressive approach (Note: Not recommended for amateur PC users) by downloading and replacing your appropriate o.js file version. We maintain a comprehensive database of 100% malware-free o.js files for every applicable version of Calibre. Please follow the steps below to download and properly replace you file:

1. Locate your Windows operating system version in the list of below "Download o.js Files".
2. Click the appropriate "Download Now" button and download your Windows file version.
3. Copy this file to the appropriate Calibre folder location:
   
   Windows 10: C:\Program Files (x86)\Calibre2\app\resources\mathjax\jax\input\MathML\entities\
   
4. Restart your computer.

If this final step has failed and you're still encountering the error, you're only remaining option is to do a clean installation of Windows 10.

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