Type Guards & Discriminated Unions

Type Guards

A type guard narrows a union type to a specific member.

function isString(x: unknown): x is string {
  return typeof x === "string";
}

function foo(val: string | number) {
  if (isString(val)) {
    // val is string here
    console.log(val.toUpperCase());
  } else {
    // val is number here
    console.log(val.toFixed(2));
  }
}

in Operator Guard

type Cat = { kind: "cat"; meow(): void };
type Dog = { kind: "dog"; bark(): void };
type Pet = Cat | Dog;

function speak(pet: Pet) {
  if ("meow" in pet) {
    pet.meow();
  } else {
    pet.bark();
  }
}

Discriminated Unions (tagged unions)

type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "square"; side: number };

function area(s: Shape): number {
  switch (s.kind) {
    case "circle": return Math.PI * s.radius ** 2;
    case "square": return s.side ** 2;
  }
}