User-Defined Type Guards

interface Cat { kind: 'cat'; meow(): void }
interface Dog { kind: 'dog'; bark(): void }
type Animal = Cat | Dog;

function isCat(a: Animal): a is Cat {
  return a.kind === 'cat';
}

function makeSound(a: Animal) {
  if (isCat(a)) {
    a.meow();          // a is narrowed to Cat
  } else {
    a.bark();          // a is narrowed to Dog
  }
}

// Works for arbitrary "is non-null" checks too:
function isPresent<T>(x: T | null | undefined): x is T {
  return x !== null && x !== undefined;
}

const nums: number[] = [1, null, 2, undefined, 3].filter(isPresent);
// nums: number[] — null/undefined filtered out at the type level