[mirekrusin]

You don't need to implement it on your own, you can use library.

Nice error messages exist there as well.

If you're casting untyped results, you can change one side and not the other and find out about this problem when in production. Or simply any mistake will get unnoticed.

Using typescript first library allows you to do much more - supports opaque types, custom constructors and any imaginable validation that can't be expressed in json schema.