The assumption here is that there are unit tests that cover the areas containing these sorts of bugs. It can prove to be false because many projects don't have

The assumption here is that there are unit tests that cover the areas containing these sorts of bugs. It can prove to be false because many projects don't have sufficiently good code coverage. Of course, it's also possible that there simply are no unit tests at all. In those circumstances, TypeScript might just result in less bugs overall. While one could argue that unit testing should be commonplace, reality doesn't always live up to that standard.