The left sidebar and the top personal toolbar can be customized by editing skin.js or common.js. skin.js applies to only the current skin, where common.js applies to all skins.
mw.util.addPortletLink: the ResourceLoader function to add links to the portlets.
portletId: portlet id— the section where the new menu item is to be placed. Valid values:
p-navigation: Navigation section in left sidebar
p-interaction: Interaction section in left sidebar
p-tb: Toolbox section in left sidebar
p-coll-print_export: Print/export section in left sidebar
p-lang: Languages section in left sidebar
p-wikibase-otherprojects: In other projects section in left sidebar
p-personal Personal toolbar at the top of the page
p-views Upper right tabs in Vector only (read, edit, history, watch, etc.)
p-cactions Drop-down menu containing move, etc. (in Vector); subject/talk links and action links in other skins
href: Link to the Wikipedia or external page
text: Text that displays
id: HTML id (optional)
tooltip: Tooltip to display on mouseover (optional)
accesskey: Shortcut key press (optional)
nextnode: Existing portlet link to place the new portlet link before (optional)
The optional fields must be included in the above order. To skip a field without changing it, use the value null.
Href
Links to Wikipedia pages are of the form '/wiki/page name'; example: '/wiki/Special:NewPages'.
External links are formatted using the full URL; example: 'http://example.org'.
There are a number of configuration variables that can be used to create more complex links. A configuration variable is read using the function mw.config.get('variableName'):
wgArticlePath: Local path, starting at the root, to reference articles, containing a "$1" placeholder that may be replaced by a page title to get a valid URL to that page.
wgPageName: The full name of the page, including the localized namespace name, if the namespace has a name (the main namespace (number 0) doesn't), and with blanks replaced by underscores.
wgServer: The server URL, not terminated by "/".
Given a valid page title title, a valid URL may be constructed using mw.config.get('wgArticlePath').replace('$1', title)