Types let you limit the size of the input space - JS lets you go ahead with (foo, bar) where they could be literally anything and you have to handle every edge

Types let you limit the size of the input space - JS lets you go ahead with (foo, bar) where they could be literally anything and you have to handle every edge case to test that (and I bet if you fuzz you'd find stuff) your unit tests don't handle. With a typed language I can specify the input space and know with certainty anything that calls that function (and compiles) is going to be within that space. This lets you dramatically reduce the complexity of your app, tests, and stress of refactoring! Fwiw I'm an SDET and I regularly find bugs, on a daily basis, in a startup of around 40 people with a Python/JS web stack that TypeScript would have caught (and am pushing towards moving over to it).