branded_types.ts

type Email = string & { __brand: "Email" };
const email = "[email protected]";

function isValidEmail(input: string): input is Email {
  return /@/.test(input);
}

function sendEmail(email: Email) {
  console.log(email);
  // send email
}

// sendEmail('not an email');
if (isValidEmail(email)) {
  sendEmail(email);
}