Integer value comparison, minimum length array, useful?

[alita-moore]

The following is an exploration into the limitations of typescript's ability to perform math comparisons. The primary limitation I found was the recursion depth being limited to 45. I don't think this is particularly practical because of this limitiation, but what do you think? Is there anything useful in the following?

// These are particularly useful when going from string -> number as to make this useful
type OneHundredNumber= [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99];
type OneTOOneHundred = "0"|"1"|"2"|"3"|"4"|"5"|"6"|"7"|"8"|"9"|"10"|"11"|"12"|"13"|"14"|"15"|"16"|"17"|"18"|"19"|"20"|"21"|"22"|"23"|"24"|"25"|"26"|"27"|"28"|"29"|"30"|"31"|"32"|"33"|"34"|"35"|"36"|"37"|"38"|"39"|"40"|"41"|"42"|"43"|"44"|"45"|"46"|"47"|"48"|"49"|"50"|"51"|"52"|"53"|"54"|"55"|"56"|"57"|"58"|"59"|"60"|"61"|"62"|"63"|"64"|"65"|"66"|"67"|"68"|"69"|"70"|"71"|"72"|"73"|"74"|"75"|"76"|"77"|"78"|"79"|"80"|"81"|"82"|"83"|"84"|"85"|"86"|"87"|"88"|"89"|"90"|"91"|"92"|"93"|"94"|"95"|"96"|"97"|"98"|"99";
type OneToOneHundredNumber = 0|1|2|3|4|5|6|7|8|9|10|11|12|13|14|15|16|17|18|19|20|21|22|23|24|25|26|27|28|29|30|31|32|33|34|35|36|37|38|39|40|41|42|43|44|45|46|47|48|49|50|51|52|53|54|55|56|57|58|59|60|61|62|63|64|65|66|67|68|69|70|71|72|73|74|75|76|77|78|79|80|81|82|83|84|85|86|87|88|89|90|91|92|93|94|95|96|97|98|99

// Having anything greater than 45 isn't practical because it exceed typescripts depth limit
type FortyFive = '0|1|2|3|4|5|6|7|8|9|10|11|12|13|14|15|16|17|18|19|20|21|22|23|24|25|26|27|28|29|30|31|32|33|34|35|36|37|38|39|40|41|42|43|44|45|end'

type ListOfNumbers = FortyFive;

// Just some helpers
type Decompose<S> = S extends `${infer _}|${infer Num}` ? [_, Num]  : never;
type ToString<T> = T extends number ? `${T}` : never;
type ToNumber<T extends OneTOOneHundred> = OneHundredNumber[T]

// max for LessThanThis is 45, ToBeCompared can be any integer
type LessThan<ToBeCompared extends number | string, LessThanThis extends number | string, Current=ListOfNumbers> =
  ToString<ToBeCompared> extends Decompose<Current>[0]
    ? ToString<ToBeCompared> extends ToString<LessThanThis>
        ? false
        : true
    : ToString<LessThanThis> extends Decompose<Current>[0]
      ? ToString<LessThanThis> extends Decompose<Current>[0]
        ? false
        : true
      : Decompose<Current> extends never
        ? false
        : LessThan<ToBeCompared, LessThanThis, Decompose<Current>[1]>

// max for GreaterThanThis is 45, ToBeCompared can be any integer
type GreaterThan<ToBeCompared extends number | string, GreaterThanThis extends number | string> = LessThan<GreaterThanThis, ToBeCompared>

// is 1 greater than 2?
type Test1 = GreaterThan<1, 2> // -> false
// is 2 greater than 2?
type Test2 = GreaterThan<2, 2> // -> false
// is 3 greater than 2?
type Test3 = GreaterThan<3, 2> // -> true

// is 2 less than 1?
type Test4 = LessThan<2, 1> // -> false
// is 1 less than 1?
type Test5 = LessThan<1, 1> // -> false
// is 0 less than 1?
type Test6 = LessThan<0, 1> // -> true

///////////// Building off of this for a practical use case...
// @ts-expect-error ignore because typescript doesn't allow for minutia
type DecomposedNumber<S> = ToNumber<Decompose<S>[0]>
// max is 44 for LessThanThis and RangeToReturn can be any length 0 | 1 | 2 ... | infinity
type AllGreaterThan<LessThanThis extends number | string, RangeToReturn extends number, Current = ListOfNumbers> =
  ToString<LessThanThis> extends Decompose<Current>[0]
    ? Exclude<RangeToReturn, DecomposedNumber<Current>>
    : AllGreaterThan<LessThanThis, Exclude<RangeToReturn, DecomposedNumber<Current>>, Decompose<Current>[1]>

// max is 44 for first, secondary can be any size
type G = AllGreaterThan<44, OneToOneHundredNumber>

type ArrayLengthMutationKeys = 'splice' | 'push' | 'pop' | 'shift' | 'unshift';
export type MinLengthArray<Element, Length extends 0 | 1 | 2 | 3 | 4, ArrayPrototype = [Element, ...Element[]]> = Pick<
	ArrayPrototype,
	Exclude<keyof ArrayPrototype, ArrayLengthMutationKeys>
> & {
	[index: number]: Element;
	[Symbol.iterator]: () => IterableIterator<Element>;
	readonly length: Length | AllGreaterThan<Length, OneToOneHundredNumber>;
};

// Does not error
const example1: MinLengthArray<string, 3> = ["1", "2", "3", "4", "6"]
// Does not error
const example2: MinLengthArray<string, 3> = ["1", "2", "3"]
// Does error (2 is not in 3 | 4 | 5 ... 99, based on my input above)
const example3: MinLengthArray<string, 3> = ["1", "2"]

[alita-moore]

There an easier way to do the min length array.

[string, ...string[]]

Which is much better. Notably though it can't be used in comparisons / only works if the type is being defined in the place it is. I think this is relatively new so maybe a bug

[sindresorhus]

It's a cool exploration of TS, but I don't think it's very practical. You should definitely write a blog post about it. Happy to link to it from here if you do.

We could also maybe add [string, ...string[]] as a tip to the readme.

[sindresorhus]

What I think we could add is type NonEmptyArray<T> = [T, ...T[]];. I would use that sometimes. Maybe also for other types like object, set, and map.

[alita-moore]

the problem I think with the NonEmptyArray in its invocation that you describe (although it also applies to one I did) is that it doesn't transfer over well to inference. For example

const foo = (bar: [string, ...string[]]) => {}

foo(["bar"]) // no error
const bar = ["bar"];
foo(bar) // Argument of type 'string[]' is not assignable to parameter of type '[string, ...string[]]'

Typescript's arbitrary type declarations and then literal comparison is the bane of my existence, lol. I spend way too much time trying to figure out ways around it. Any ideas on a work around? I'll create a ticket in typescript in the meantime though.