The type isn't just there to make it easy to understand when you do it, it's for you a year later when you need to make a change further inside a codebase, far from where it's validated. Or for someone else who's never even seen the validation section of code.
I'm hung up on the type system because it's a great way to convey the validity of the data; it follows the data around as it flows through your program.
I don't (yet) Typescript, but jsdoc and linting give me enough type checking for my needs.
