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User JavaScript requires that you are using Opera 8.0 or higher

Configuring Opera

Before User JavaScript can run in Opera, you must enable the feature by pointing Opera to your User JavaScript directory:
1. Create a directory for storing User JavaScript files, for instance C:\userjs\
Go to Tools -> Preferences -> Advanced -> Content -> JavaScript Options. Add the location of your newly created script directory to My JavaScript files.
Some scripts might require that you enable an additional option in your opera6.ini file. If you started using User JavaScript in Opera 8.0, then enabling this feature is strongly recommended. In Opera 8.01 and later, this feature is enabled automatically when you select your User JavaScript directory:
2. Locate your opera6.ini file by viewing the About Opera page, where the location is found under Path -> Preferences.
3. Close Opera.
4. Open the opera6.ini file located earlier in a text editor, and add the option Always Load User JavaScript=1 to the [User Prefs] section.
5. Reopen Opera.

Installing User JavaScript files

On the description page for each individual script, there is a link that says "Download User JS". Scripts can be installed in one of the following ways:
If you want to review the script prior to installation, you can just visit the script, and select "Save As" when you are viewing the script.
Mouse: By right-clicking the download link, and select "Save target as" . Save the script to your previously created User JavaScript folder.
Keyboard: By focusing the download link, and pressing "Ctrl-Shift-S". Save the script to your previously created User JavaScript folder.

Uninstalling User JavaScript files

Permanent uninstallation of User JavaScript files is done by deleting the script file from the folder containing your User JavaScript files.
If you want to disable a script file without permanently removing it, this is easiest achieved by changing the file suffix of the script file. Example: renaming myScript.js to myScript.js.d will disable the script.


Font: http://userjs.org/help/installation