As a daily user of IntelliJ IDEA—which, IME, has the best autocomplete and code suggestions of any IDE for dynamically typed languages—there is a world of a difference between the quality of suggestions for a codebase using TypeScript and one written in vanilla JS. Without concrete types, the IDE often has to guess the possible type(s) that a value can have, whereas with TS, there is (generally) no such ambiguity.

I don't really experience this issue and I use Web storm which is basically the same thing. I find that the dynamic type hinting plus the combination of using JS Doc formatted comments rarely leaves me with any ambiguity when writing vanilla JavaScript.

Are you using types in your JS Doc comments? If so, that's not vanilla JavaScript.

It is because it runs in the browser unprocessed.

You edit a file and reload, no munging pipeline,

JS Doc is just comments. (But the typechecker can still read them.)