JavaScript

JavaScript is a powerful scripting language used by web browsers and can be used by administrators of wikis to add features, functionality or style to their wikis. This scripting language is executed in the browser rather than on the server, so it results in less load on the wiki server for implementing features. There are limitations around any features that require storage, especially if information needs to stay confidential. Since JavaScript can be used to edit wiki pages it is possible to store formatted data within a page but it can be vulnerable to manipulation. JavaScript can interact with the MediaWiki API to gather data that can be used for purposes such as generating custom logs.

Bot scripting
One powerful use of JavaScript is the creation of bot scripts that function within the browser. Many administrators may wish to use bots but installing the required software and configuring it correctly can be an insurmountable challenge to users who don't already have the required technical experience. On-wiki bots written in JavaScript that use pages for their interface are much more accessible.

Security warning
JavaScript can be misused by malicious users, and any administrator by default can add and edit JavaScript on their wiki. Be very careful about adding any JavaScript based on user requests. MediaWiki and Miraheze do appear to have some safeguards to protect against compromising account but JavaScript could still potentially be used to gain access to accounts or personal information stored within accounts. JavaScript can be used to make edits, so it's possible that a malicious user could engineer a situation where JavaScript is added to a wiki that allows them to impersonate a user. Be very careful if implementing a JavaScript bot that makes edits based on edits to a user's subpages, as a malicious user could possibly exploit this to impersonate other users by forcing them to make edits.