Greasemonkey

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For the slang term referring to mechanics see Grease monkey. For the comic see Grease Monkey (comics).

Screenshot of the BookBurro user script running in Greasemonkey. BookBurro alters an amazon.com page to show the prices of the same book offered by competing retailers.

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Greasemonkey is a Mozilla Firefox extension that allows users to install scripts that make on-the-fly changes to specific web pages. As the Greasemonkey scripts are persistent, the changes made to the web pages are executed every time the page is opened, making them effectively permanent for the user running the script.

Greasemonkey can be used for adding new functionality to web pages (for example, embedding price comparison in Amazon.com web pages), fixing rendering bugs, combining data from multiple webpages, and numerous other purposes. Well written Greasemonkey scripts can integrate changes so well that their additions appear to be natural parts of the web page.

1 Technical details
2 Typical Greasemonkey scripts
3 Technical, operational, and ethical issues arising from user scripting
4 Greasemonkey compatibility and equivalents for other browsers
5 Similar software
6 Alternatives
7 See also
8 External links
9 External reviews

[edit] Technical details

Most Greasemonkey user scripts are written by hand, using site-specific JavaScript code which manipulates the contents of a web page using the Document Object Model interface. userscripts.org maintains a database of Greasemonkey scripts, and for each it lists the URLs of web pages to which the scripts pertain. (N.B. The obsolescent script repository might contain some older scripts that have not yet migrated to the new repository.) When the user loads a matching page Greasemonkey invokes the relevant scripts, which can then add to the page, delete parts of it, or move parts around. Greasemonkey scripts can also poll external HTTP resources via a non-domain-restricted XMLHTTP request. Greasemonkey scripts have the format somename.user.js, and Greasemonkey automatically detects and offers to install any such scripts which it encounters. In addition to JavaScript code, Greasemonkey scripts contain limited optional metadata, which specifies the name of the script, a description, a namespace URL used to differentiate identically named scripts, and the default list of URLs for which the script is intended.

Writing a Greasemonkey script is a technically demanding process (although rather easier than writing a full-fledged Firefox extension). This means that few ordinary users can practically write Greasemonkey extensions. The Platypus [1] extension, however, allows users to edit a page (deleting parts of it, or moving parts around); Platypus then saves these changes as a persistent Greasemonkey script.

[edit] Typical Greasemonkey scripts

Users have written scripts which:

Alter Gmail to embed Google Reader into it, thus providing an RSS feed option.
Show a list of competing retailers' prices for a book when viewing the book on online retailers
Remove advertisements from many sites, including popups and google text ads.
Alter the layout of pages.
Autofill forms.
Add or remove accessibility features from pages.
Let a user save an FLV file from popular video streaming sites such as Google Video and YouTube.

[edit] Technical, operational, and ethical issues arising from user scripting

Some in the Firefox community, and a number of technical analysts, warn that widespread use of Greasemonkey and related user scripting technologies will require care in deployment. Their concerns include:

Possible disruption to revenue of some websites. Some Firefox users already block web advertisements with Adblock Plus, but Greasemonkey may be used to change ads from one company to those of their competitor, to artificially inflate web advertising scores, or to fool the statistics programs run by paid browsing companies.
Increases in the web traffic that arise from every page. Some Greasemonkey scripts obtain additional information about pages the user visits, for example from services such as del.icio.us and Bloglines. If these scripts were widely used, they would cause substantial additional traffic to the target websites.
The proliferation of scripts acting on many pages, and the difficulty of distinguishing whether problems in a page are caused by actual web-programming bugs or by local Greasemonkey scripts, may make troubleshooting defects more difficult.

[edit] Greasemonkey compatibility and equivalents for other browsers

Greasemonkey is available for Firefox, Flock and Epiphany. The Epiphany Greasemonkey extension is part of the Epiphany-extensions package. However, this extension is not fully compatible as of release 2.15.1, since some Greasemonkey API functions (e.g. GM_getValue) are unsupported. There is also a custom version for SeaMonkey: [2]

Version 8 of Opera also adds user scripting functionality. As both Opera and Firefox support the W3C DOM, many Greasemonkey user scripts also work correctly on Opera.

Creammonkey and PithHelmet (shareware) are similar tools for the Safari browser.

For Internet Explorer, Trixie, IE7pro and Turnabout offer similar functionality. Turnabout used to be open source software (under the BSD License), but as of September 2006, the source code was no longer available.

[edit] Similar software

Proxomitron, available since late 1990s, predates JavaScript active browsing and provides similar functionality for all browsers using a regexp-like matching language.
Proximodo, inspired by- and interoperable with proxomitron, is an open source, client side Proxy Server which allows manipulation of page content
Privoxy is an open source, client side Proxy Server which allows manipulation of page content available in many OS flavors
MouseHole is a client side Proxy Server which allows manipulation of page content using Ruby.
Monkeygrease is a Java Servlet which can be used to alter the output of a closed-source Java web application before its output is sent to the client.
Chickenfoot is a Firefox extension that aims to enable both end-user programmers and hackers to script interactions on the Web.
iMacros for Firefox is a Firefox extension that allows the user to record and replay so called "Internet Macros" for web automation, web scraping or web testing.
Stylish is a Firefox extension that allows for client-side manipulation of webpage content through Cascading Style Sheets.
Bookmarklets can execute arbitrary JavaScript on any page, but they require a user to click them, rather than running automatically.

[edit] Alternatives

Without an extension like Greasemonkey, modifications to websites can also be done in one of the following ways:

Entering javascript: URLs in the Location Bar or using bookmarklets
Using the browser's DOM Inspector
Using a local HTTP proxy that modifies the HTML, such as WebWasher or Proxomitron
Using the Opera browser and user .js scripts