Typescript is basically required to keep you from shooting yourself in the foot with Javascript.
Type annotation also serves as a form of 'hard' documentation. If it's wrong or no longer correct, it will make things turn orange in your IDE and make your type linting tool go bonkers. Even as a single developer this has tremendous benefits. It's much easier to come back to something you wrote months/years ago and get back into it rather than having to go through each function to see what it returns under every situation.
Type annotation also serves as a form of 'hard' documentation. If it's wrong or no longer correct, it will make things turn orange in your IDE and make your type linting tool go bonkers. Even as a single developer this has tremendous benefits. It's much easier to come back to something you wrote months/years ago and get back into it rather than having to go through each function to see what it returns under every situation.