As a Wikipedia user script/gadget developer, I would like to have [[https://www.typescriptlang.org/docs/handbook/declaration-files/introduction.html|TypeScript declaration files]] for the MediaWiki Javascript API, so that developing is possible without having to constantly check the [[https://doc.wikimedia.org/mediawiki-core/master/js/#!/api/mw|MediaWiki Documentation]].
TypeScript declaration files are supported by many [[https://en.wikipedia.org/wiki/Integrated_development_environment|IDEs]], which can provide type checking and autocomplete for user scripts. An example of a supporting IDE is [[https://code.visualstudio.com/docs/languages/typescript#_intellisense|VS Code]].
Currently, MediaWiki is using JSDuck, and when the migration to JSDoc (T138401) is completed, declaration files [[https://www.typescriptlang.org/docs/handbook/declaration-files/dts-from-js.html|can be automatically generated from the JSDoc]].
If MediaWiki [[https://www.typescriptlang.org/docs/handbook/declaration-files/publishing.html|publishes]] the typings after generation, users like me are able to [[https://www.typescriptlang.org/docs/handbook/declaration-files/consumption.html|download and use]] the typings.