Essentials of Javascript - Cultural View

Bookmarklet 60
Bookmarklet
A bookmarklet is an applet, a small computer application, stored as the URL of a bookmark in a web browser or as a
hyperlink on a web page. The term is a portmanteau of the terms bookmark and applet. Whether bookmarklet
utilities are stored as bookmarks or hyperlinks, they are designed to add one-click functionality to a browser or web
page. When clicked, a bookmarklet performs some function, one of a wide variety such as a search query or data
extraction. Usually the applet is a JavaScript program.
Concept
Web browsers use URIs for the href attribute of the tag and for bookmarks. The URI scheme, such as http:, file:,
or ftp:, specifies the protocol and required form for the rest of the string. Browsers also implement a prefix
javascript: that to a parser is just like any other URI. Internally, the browser sees that the protocol is javascript, treats
the rest of the string as javascript code which is then executed, and uses the resulting string as the new page.
The executing script has access to the current page, which it may inspect and change. If the script returns an
undefined type (rather than, say, a string), the browser will not load a new page, with the result that the script simply
runs against the current page content. This permits in-place font size and color changes, for example, without a page
reload.
An anonymous function can be used to force the script to return an undefined type:
javascript:(function(){
/* Statements returning a non-undefined type, e.g. assignments */
})();
Usage
Bookmarklets are saved and used as normal bookmarks. As such, they are simple "one-click" tools which add
functionality to the browser. For example, they can:
• Modify the appearance of a web page within the browser (e.g., change font size, background color, etc.).
• Extract data from a web page (e.g., hyperlinks, images, text, etc.).
• Submit the current page to a blogging service such as Posterous, link-shortening service such as su.pr, or
bookmarking service such as Delicious.
• Query a search engine, with search term(s) provided by previously selected text, or by a dialog box.
• Submit the current page to a link validation service, or translation service.
• Set commonly chosen configuration options when the page itself provides no way to do this.
"Installation of a bookmarklet" is performed by creating a new bookmark, and pasting the code into the URL
destination field. Alternatively, if the bookmarklet is presented as a link, under some browsers it can be dragged and
dropped onto the bookmark bar. The bookmarklet can then be run by loading the bookmark normally.