I disagree, especially in the realm of AST-crawling, that having type hints in the IDE has been extremely beneficial in moving fast.
Type-level refactoring tools don't seem to be very powerful at the moment, too; without them, you can end up with some legwork when you change a design decision (a frequent thing while prototyping), even if you carefully aliased every type (which is legwork in its own right, because trivial cases like 2-tuples are plentiful).
Fortunately, TS allows you to gradually type your code, and rely on inference in many cases (not as much as Haskell, but still).
Bonus points if it comes in notebook form so you get a nice convenient run button, a solid multiline text editor, and markdown-powered persistence format.
Thankfully, this already exists: https://marketplace.visualstudio.com/items?itemName=jakearl.... . And it supports every language your shell does!
(I made it)
https://www.typescriptlang.org/docs/handbook/jsdoc-supported...
Since the new ESLint is going to be highly modular, TS would shine in defining these modules' interfaces in a robust way. Whether to use TS deeper down, or whether to enforce the use of TS across the codebase, is another question. Prototyping is often easier and faster with plain JS, while typing well-defined parts later may be helpful for maintenance purposes.