Wikipedia:User script sandbox/Installation
To install the user script sandbox user script, add the following code to your skin.js or common.js page:
mw.loader.load( "//en.wikipedia.org/w/index.php?title=User:PleaseStand/userScriptSandbox.js&action=raw&ctype=text/javascript" );
Bypass your browser's cache if you need to, and then go to Project:User script sandbox to edit the contents of your local sandbox, which are not saved to a wiki page. To make this script work in wikis other than Wikipedia, copy the contents of Project:User script sandbox into a page with the same name in your wiki's Project:
namespace; the editor will be available on that page.
This script's source code is at User:PleaseStand/userScriptSandbox.js, as PleaseStand is its current maintainer. Be sure to leave a message if you like the script, have suggestions for improvement, or simply want to use it elsewhere.
Alternative version
A semi-experimental fork of the script is located at User:Kephir/gadgets/jssand.js. It adds the Ace editor, which allows you to type tabs and provides syntax highlighting and hints, and sends a confirmation dialog if you try to close the script sandbox with unsaved changes. Install it with this line:
mw.loader.load( "//en.wikipedia.org/w/index.php?title=User:Kephir/gadgets/jssand.js&action=raw&ctype=text/javascript" ); // [[User:Kephir/gadgets/jssand.js]]