A nominal type system means that each type is unique and even if types have the same data you cannot assign across types. TypeScript's type system is structural, which means if the type is shaped like a duck, it's a duck. If a goose has all the same attributes as a duck, then it also is a duck. You can learn more here: example:structural-typing This can have drawbacks, for example there are cases where a string or number can have special context and you don't want to ever make the values transferrable. For example: - User Input Strings (unsafe) - Translation Strings - User Identification Numbers - Access Tokens We can get most of the value from a nominal type system with a little bit of extra code. We're going to use an intersectional type, with a unique constraint in the form of a property called __brand (this is convention) which makes it impossible to assign a normal string to a ValidatedInputString.

type ValidatedInputString = string & { __brand: "User Input Post Validation" };

// We will use a function to transform a string to a ValidatedInputString - but the point worth noting is that we're just _telling_ TypeScript that it's true.

const validateUserInput = (input: string) => {
  const simpleValidatedInput = input.replace(/\
// Now we can create functions which will only accept
 our new nominal type, and not the general string type.

const printName = (name: ValidatedInputString) => {
  console.log(name);
};


// For example, here's some unsafe input from a user, going
 through the validator and then being allowed to be printed:

const input = "alert('bobby tables')";
const validatedInput = validateUserInput(input);
printName(validatedInput);


// On the other hand, passing the un-validated string to
 printName will raise a compiler error:

printName(input);


// You can read a comprehensive overview of the
 different ways to create nominal types, and their
 trade-offs in this 400 comment long GitHub issue:

 https://github.com/Microsoft/TypeScript/issues/202

 and this post is a great summary:

 https://michalzalecki.com/nominal-typing-in-typescript/