block6.ts

function isUser(o: any): o is User {
  return o && !!o.firstName && !!o.lastName && !!o.age;
}

const maybeUser: any = null;

if (isUser(maybeUser)) {
  //We don't have to cast maybeUser
  //TypeScript know that in this 'if', maybeUser is a User
  console.log(maybeUser.firstName);
}