IsNever
Challenge
Implement a type IsNever<T>, which takes input type T. If the type of T
resolves to never, return true, otherwise false.
For example:
type A = IsNever<never>; // expected to be true
type B = IsNever<undefined>; // expected to be false
type C = IsNever<null>; // expected to be false
type D = IsNever<[]>; // expected to be false
type E = IsNever<number>; // expected to be false
Solution
The most obvious solution here is to check if the type is assignable to never
with the help of conditional types. If we can assign the type T to never, we
return true, otherwise false.
type IsNever<T> = T extends never ? true : false;
Unfortunately, we do not pass the test case for never itself. Why is that?
extends becomes distributive when the left-hand side is a generic and a union
is supplied through it. never represents an empty union, so when never is
supplied through a generic, nothing gets distributed to the right-hand side of
extends, and the whole thing resolves to never, i.e. an empty union.
To avoid this distributivity, we can surround each side of extends with square
brackets. This way, the left-hand side of extends becomes a tuple holding a
generic rather than a generic itself.
type IsNever<T> = [T] extends [never] ? true : false;
With this workaround, hack, genuine solution, you name it; we can pass the test
and implement a generic type to check if the type is never.
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