Improve Runtime Type Safety with Branded Types in TypeScript

branded.ts

declare const __brand: unique symbol
type Brand<B> = { [__brand]: B }
export type Branded<T, B> = T & Brand<B>

/**
 * Branded type for a person's age, should belong to the interval [0, 125] (inclusive)
 **/
type Age = Branded<number, "Age">;

/**
 * Validates the input number as a valid age, throws an error if it is not.
 **/
function createAge(input: number): Age {
  if (input < 0 || input > 125) {
    throw new Error("Input value for Age is not within the valid range")
  }

  return input as Age;
}

function getBirthYear(age: Age) {
    return (new Date()).getFullYear() - age
}

/** Usage **/


const myAge: Age = createAge(36); // true
const birthYear = getBirthYear(myAge) 

const birthYear2 = getBirthYear(36)  // This should show an error