3.js

type Animal =
  | { t: 'Cat' }
  | { t: 'Dog' }
;

function makeSound<T: Animal>(arr: Array<T>) {
  arr.forEach(animal => {
    if (animal.t === 'Cat') {
      console.log('meow');
    } else {
      console.log('woof');
    }
  });
}