Yeah don't do that. :)

Do this:

  interface Dog { typeName: "Dog"; woof():void }
  interface Cat { typeName: "Cat"; meow():void }
  
  function isDog(a:Dog|Cat) : a is Dog {
    return a.typeName == "Dog"; // Some casting may be required here
  }
  
  function f(a: Dog|Cat) {
    if (isDog(a)) a.woof();
    else a.meow();
  }
  
  let dogish : Dog = {typeName:"Dog", woof: ()=>{ console.out("Woof this is Dog")}};
  f(dogish);

The neat thing about TypeScript's type system is that it's structural but you can pretty easily implement nominal types on top of it.

(And if you only need compile-time checks, you can make typeName nullable; {typeName?:"Dog"} != {typeName?:"Cat"})