Union types and type narrowing in TypeScript

Union types

A union type accepts any of its members. TypeScript tracks which branch of a conditional is taken — this is control flow analysis:

function format(input: string | number): string {
  if (typeof input === "string") {
    return input.toUpperCase(); // input is string here
  }
  return input.toFixed(2); // input is number here
}

instanceof narrowing

function process(value: Date | string) {
  if (value instanceof Date) {
    return value.toISOString(); // Date
  }
  return value;                // string
}

in operator narrowing

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

function makeNoise(animal: Cat | Dog) {
  if ("meow" in animal) {
    animal.meow(); // Cat
  } else {
    animal.bark(); // Dog
  }
}

TypeScript's control flow analysis handles all standard narrowing patterns. After an if block that returns or throws, the type in the else branch is automatically narrowed — you do not need explicit else when the if branch exits.