Earlier quoted context omitted.
Looking back and forth between code and docs just to remember the shape of returned objects is annoying and probably the worst for productivity. I imagine most people using vanilla JS have significant amounts of objects memorized.
IDEs like webstorm can ostensibly be quite helpful if you use jsdoc style annotations. However, in my experience, once a code base is big enough, you can open almost any file and find documentation that's out of date, incorrectly copy-pasted, or some other major issue. TS, on the other hand, at least forces you to stay correct, which is ultimately what convinced me to switch.
Which are a form of type annotations. Case in point: the typescript compiler can actually parse those, in .js files, and yield appropriate warnings on errors. See the //@ts-check annotation.