Branded types with TypeScript

branded-types.ts

/**
 * @source https://github.com/mike-works/typescript-fundamentals/blob/4e5782e00ed942ffe8770a694122190b6ed69b95/notes/6-guards-and-extreme-types.ts
 * Helps guard against accidentally matching a type that might otherwise be easy to mismatch
 */

// Brand A
interface BrandedA {
  __this_is_branded_with_a: "a"; // some unique primitive brand
}

function brandA(value: string): BrandedA {
 return (value as unknown) as BrandedA;
}

function unbrandA(value: BrandedA): string {
 return (value as unknown) as string;
}

// Brand B
interface BrandedB {
  __this_is_branded_with_b: "b";
}

function brandB(value: { abc: string }): BrandedB {
  return (value as unknown) as BrandedB;
}

function unbrandB(value: BrandedB): { abc: string } {
  return (value as unknown) as { abc: string };
}

let secretA = brandA("This is a secret value");
let secretB = brandB({ abc: "This is a different secret value" });

secretA != secretB; // ✅ No chance of getting these mixed up
unbrandB(secretA);
unbrandA(secretB);

// back to our original values
let revealedA = unbrandA(secretA);
let revealedB = unbrandB(secretB);

// 💡 PROTIP - always brand/unbrand casting in exactly one place.