Typescript will pick up the JSDoc type annotations and use them to type the code. This can be a great option when typing an existing project.
Docs: https://www.typescriptlang.org/docs/handbook/type-checking-j...
Recently, Typescript was complaining about an implicit `any` even though the VSCode tooltip clearly could infer the actual type (it confusingly was showing both cases in the tooltip). I finally realized that VSCode's built-in inference worked here but Typescript didn't work at all because I forgot I was importing a .js file rather than a .ts file.
VSCode for course also downloads @types/* files in the background even in vanilla JS projects which is super helpful because you get effortless intellisense. That it does this kind of stuff out of the box is the sort of reason why I've lost my Vim/Emacs fanaticism over the years.
VSCode tries to give you type info in contexts where Typescript wouldn't (e.g. importing Javascript files, in a completely non-TS project) so it makes sense that its inference tooltip can be more helpful. You can easily recreate this by importing a fn from a JS module and then hovering it in VSCode (also configured for Typescript): VSCode knows the fn signature but Typescript won't. Both cases show up in the tooltip (though confusingly).
At least, that's my interpretation of this sort of tooltip: https://i.imgur.com/lXS5teK.png
And to be clear, your last sentence was my exact praise for VSCode at the end of my own comment.
Are these coming from DefinitelyTyped?
It doesn't auto-download @types, but it does suggest them to you. It can also auto-import, rename across files, etc.
Re-using types (structs or union types) is also a huge pain.