ValueType example in typescript

ValueType.ts

const typeKey = Symbol('type'); 

enum ValueTypeDef {
  YEAR = "YEAR",
  NAME = "NAME",
  TITLE = "TITLE"
}

type ValueType<K extends ValueTypeDef, T> = {
  [typeKey]: K;
  value: T;
}

interface ValueObject<K extends ValueTypeDef, T> extends Readonly<ValueType<K, T>> { }
interface Name extends ValueObject<ValueTypeDef.NAME, string> { }
interface Title extends ValueObject<ValueTypeDef.TITLE, string> { }
interface Year extends ValueObject<ValueTypeDef.YEAR, number> { }

const ValueType = <K extends ValueTypeDef, T>(type: K, value: T): ValueObject<K, T> => ({ [typeKey]: type, value: value })

function yearOf(value: number): Year {
  if (value < 1600) throw new Error("Year must be greater than 1900")
  return ValueType(ValueTypeDef.YEAR, value);
}
const nameOf = (value: string): Name => ValueType(ValueTypeDef.NAME, value);
const titleOf = (value: string): Title => ValueType(ValueTypeDef.TITLE, value);

/// implementation examples

const value = "My Book Title";

const age1: Title = { type: ValueTypeDef.TITLE, value }; // not allowed because variable is typed
const title2 = { type: ValueTypeDef.TITLE, value }; // while this is allowed you can't use it with anything that is typed see MakeBook

let age = yearOf(1895)
//age = 2015 ... nope not a valid year even with out explicit typing of age ... TS knows its a Year
//let age3:Year = 2015 ... nope must use yearOf
//age.value = 1906 ... nope value is readonly

const author = nameOf("Rudyard Kipling");
const title:Title = titleOf("Food")


function MakeBook(author:Name, title: Title) {
  console.log( { author: author.value, title: title.value } )
  return { author: author.value, title: title.value }
}

MakeBook(author, title2) // this raises an error in TS but works in JS

MakeBook(author, title) // this is valid