That's distinguishing the String class from primitive string. I don't think that would still work with another `extends String` the same shape as Hash.

For example: https://www.typescriptlang.org/play/?#code/GYVwdgxgLglg9mABO...

  class Animal {
    isJaguar: boolean = false;
  }

  class Automobile {
    isJaguar: boolean = false;
  }

  function engineSound(car: Automobile) {
    return car.isJaguar ? "vroom" : "put put";
  }

  console.log(engineSound(42)); // TypeScript complains
  console.log(engineSound(new Animal())); // TypeScript does not complain

Or, a version that's more inline with the post you're replying to.

Just add an Email class that also extends String and you can see that you can pass an Email to the compareHash function without it complaining.

  class Hash extends String {}
  class Email extends String {}

  // Ideally, we only want to pass hashes to this function
  const compareHash = (hash: Hash, input: string): boolean => {
    return true;
  };
  
  const generateEmail = (input: string): Email => {
    return new Email(input);
  }
  
  // Example usage
  const userInput = "secretData";
  const email = generateEmail(userInput);
  
  // Whoops, we passed an email as a hash and TS doesn't complain
  const matches = compareHash(email, userInput);

https://www.typescriptlang.org/play/?#code/MYGwhgzhAEASkAtoF...

Oops you’re right!

Great example of something that does not work. Javascript classes are structural by default, Typescript does nothing there.

https://www.typescriptlang.org/play/?#code/MYGwhgzhAEASkAtoF...

and works correctly in flow [0]

[0] https://flow.org/try/#1N4Igxg9gdgZglgcxALlAIwIZoKYBsD6uEEAzt...