https://github.com/Microsoft/TypeScript/wiki/JavaScript-Lang...
> Visual Studio 2017 provides a powerful JavaScript editing experience right out of the box. Powered by a TypeScript based language service, Visual Studio delivers richer IntelliSense, support for modern JavaScript features, and improved productivity features such as Go to Definition, refactoring, and more.
It works off Typescript type annotations for packages and the Typescript compiler's type inference. VS Code's Javascript editing does not seem to be evidence for tooling around dynamic languages so much as it's evidence that if you have a lot of money and Anders Hejlsberg you can write a compiler for a statically typed language with type inference that looks like a specific dynamic language.
Are there not definitions written by someone or a tool that vscode looks at?
That is what I saw using some npm packages with typescript: http://definitelytyped.org/