Typescript Odd Number Type

#webdev #typescript #programming #tutorial

Most people: No, you cannot create an odd number type with Typescript

Meanwhile me:

type OddNumber<
    X extends number,
    Y extends unknown[] = [1],
    Z extends number = never
> = Y['length'] extends X
    ? Z | Y['length']
    : OddNumber<X, [1, 1, ...Y], Z | Y['length']>

type a = OddNumber<1> // 1 
type b = OddNumber<3> // 1 | 3
type c = OddNumber<5> // 1 | 3 | 5
type d = OddNumber<7> // 1 | 3 | 5 | 7

playground

with some limitations, the input must an odd number, and cannot exceed 1999 (maximum depth of typescript recursion is only 1000)

you can do even number type using similar logic

Top comments (4)

ecyrbe • Jun 6 '22 • Edited

Ok, challenge accepted. If you are not affraid of generic programming you can do Odd or Even type check without limitation :

type Number2String<T extends number> = `${T}`;
type IsStringEven<T extends string> = 
  T extends `${string}${0|2|4|6|8}` ? true : false;

type IsStringOdd<T extends string> = 
  T extends `${string}${1|3|5|7|9}` ? true : false;

type EvenNumber<T extends number> = 
  IsStringEven<Number2String<T>> extends true ? T : never;

type OddNumber<T extends number> = 
  IsStringOdd<Number2String<T>> extends true ? T : never;

function guarantyOddNumber<T extends number>(n: EvenNumber<T>) {
  return n+1;
}

function guarantEvenNumber<T extends number>(n: OddNumber<T>) {
  return n+1;
}

guarantyOddNumber(6876546); // succeed
guarantyEvenNumber(6876546); // fail

check on playground

Acid Coder • Jun 7 '22 • Edited

good solution, I further simplified your code

type IsEven<T extends number> = 
  `${T}` extends `${number}${0|2|4|6|8}` ? T : never;

type IsOdd<T extends number> = 
  `${T}` extends `${number}${1|3|5|7|9}` ? T : never;

function guarantyOddNumber<T extends number>(n: IsOdd<T>) {
  return n;
}

function guarantEvenNumber<T extends number>(n: IsEven<T>) {
  return n;
}

guarantyOddNumber(6876545); // succeed odd
guarantyOddNumber(6876546); // fail odd
guarantEvenNumber(6876546) // succeed even
guarantEvenNumber(6876545); // fail even