Can't use type-fest GreaterThan as parameter type

Question

I tried using type-fest GreaterThan type for the function parameter and got an error

when I write the following function:

import type {GreaterThan} from 'type-fest'

function printNumberOfBooks <N extends number> (numberOfBooks: GreaterThan<N, 0>): void {
    console.log(numberOfBooks)
}

and... :

printNumberOfBooks(2)

I get the following typescript error:

Argument of type 'number' is not assignable to parameter of type 'never'

It took me hours... I didn't find a solution

Official documentation of type-fest GreaterThan type

Official documentation of typescript Generics

Answer 1

Check this out ...

In above link as n0tKamui said:

GreaterThan is a boolean reduction, so you would need to check it

Base on this and other things like the friendly error message... The best way to implement it is: (shout-out to nadameu)

import type { GreaterThan } from 'type-fest';

function printNumberOfBooks<N extends number>(
  numberOfBooks: GreaterThan<N, 0> extends true
    ? N
    : { error: 'input is not greater than zero' }
): void {
  console.log(numberOfBooks);
}

printNumberOfBooks(2); // No error

printNumberOfBooks(-3);
//                 ^       Argument of type 'number' is not assignable to
//                         parameter of type '{ error: "input is not greater 
//                         than zero"; }'.ts(2345)