Type Predicates (is keyword) for Custom Type Guards

Functions that check a value's type return 'boolean', so TypeScript doesn't narrow the type inside the calling code's if block.

Use a type predicate return type ('param is Type') to inform TypeScript that a true return means the parameter is the specified type.

interface Cat { meow(): void; }
interface Dog { bark(): void; }

// Without predicate — no narrowing
function isCat(animal: Cat | Dog): boolean {
  return 'meow' in animal;
}

// With predicate — narrows correctly
function isCat(animal: Cat | Dog): animal is Cat {
  return 'meow' in animal;
}

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

// Filtering arrays with type predicates
const values: (string | null)[] = ['a', null, 'b'];
const strings = values.filter((v): v is string => v !== null);
// strings: string[] — not (string | null)[]

TypeScript's control flow analysis only narrows based on built-in checks (typeof, instanceof) or type predicates. A predicate annotation tells the compiler to trust the function's boolean return as a type guarantee.