As a Wikipedia user script/gadget developer, I would like to have TypeScript declaration files for the MediaWiki Javascript API, so that developing is possible without having to constantly check the MediaWiki Documentation.
TypeScript declaration files are supported by many IDEs, which can provide type checking and autocomplete for user scripts. An example of a supporting IDE is VSCode.
Currently, MediaWiki is using JSDuck, and when the migration to JSDoc (T138401) is completed, declaration files can be automatically generated from the JSDoc.
If MediaWiki publishes the typings after generation, users like me are able to download and use the typings.