Type Guards

interface Dog {
  bark(): void;
}

interface Cat {
  meow(): void;
}

function isDog(animal: Dog | Cat): animal is Dog {
  return (animal as Dog).bark !== undefined;
}

// Usage
function makeSound(animal: Dog | Cat) {
  if (isDog(animal)) {
    animal.bark(); // TypeScript knows it's Dog
  } else {
    animal.meow(); // TypeScript knows it's Cat
  }
}

function isNotNull<T>(value: T | null | undefined): value is T {
  return value !== null && value !== undefined;
}

const items = [1, null, 2, undefined, 3];
const filtered = items.filter(isNotNull); // number[]

function hasProperty<T, K extends string>(obj: T, key: K): obj is T & Record<K, unknown> {
  return typeof obj === 'object' && obj !== null && key in obj;
}