Type IsNever

[macabeus]

Search Terms

never, distributive conditional type

Suggestion

As described at this issue, this behaviour is the expected:

type IsNever<T> = (
  T extends never
    ? true
    : false
)

let foo: IsNever<1> // false
let bar: IsNever<never> // never

But since it always returning false or never is complicated to compound more complex types, for example:

type ArrayHasLessThanThreeElements<T> = (
    IsNever<Extract<'2', keyof T>>
)

let a: ArrayHasLessThanThreeElements<[1, 2, 3]> // false
let b: ArrayHasLessThanThreeElements<[1, 2]> // never, but the expected type is "true"

Use Cases

It is useful when you need to compound types and use something like T extends never.
Since if T is never the truthy clausure isn't run, it's very complicated to compound types at this case.

Examples

Let's say that we have the new utility type IsNever from TS:

type ArrayHasLessThanThreeElements<T> = (
    IsNever<Extract<'2', keyof T>>
)

let a: ArrayHasLessThanThreeElements<[1, 2, 3]> // false
let b: ArrayHasLessThanThreeElements<[1, 2]> // true

[yume-chan]

Just scroll down a little bit to #31751 (comment) you will find how to do IsNever as your Intended.

type IsNever<T> = [T] extends [never] ? true: false;

type ArrayHasLessThanThreeElements<T> = (
    IsNever<Extract<'2', keyof T>>
)

let a: ArrayHasLessThanThreeElements<[1, 2, 3]> // false
let b: ArrayHasLessThanThreeElements<[1, 2]> // true

(playground)