What do you mean by redefining? I'm not redefining the type of "name". If you want to get compile-time checking that `Person` implements `Named`, you can use `class Person implements Named`.

Even better than that, you don't need to use classes, you can use "normal" JS data structures. Extending the last example:

     const john = {
         name: 'John',
         age: 25
     } // No class involved

     f(john) // compiles

     const alice = {
         age: 25
         firstName: 'Alice',
         lastName: 'Jones'
     }

     f(alice) // compile-time error

Clojure.spec is cool, but I don't see how that is incompatible with static typing. You can still have libraries that check more complex properties at runtime.

EDIT: > Also, Clojure.spec allows you to be much more precise about properties. For example, it must be [...] non-nil [...]

TypeScript also handles nulls in the type system:

    function f(x: string | null) {
        if (x != null) {
              // tsc knows that inside this if, x can't be null
              return x.length 
        } else {
              console.log(x.length) // this doesn't compile, x is of type null here
        }
    }