Variations in comparing tuple types in TypeScript

Exploring the TypeScript Challenge, there is a particular problem known as IsNever. The task at hand is to create a type called IsNever that takes input of type T. If the resolved type equates to never, the output should be true; otherwise, it should be false.


    type A = IsNever<never> expected result: true
    type B = IsNever<undefined>  expected result: false
    type C = IsNever<null>  expected result: false
    type D = IsNever<[]>  expected result: false
    type E = IsNever<number>  expected result: false

Here is my solution (which turned out to be incorrect):

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

An alternative solution proposed:

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

What sets these two solutions apart? I am curious about the distinctions in comparing types with tuples.

Answer №1

Exploring the concept of distributive conditional types.

Imagine types as collections of values. In this context, T represents a collection of all values that can be assigned to it, while never signifies an empty collection. The gist of the first syntax is akin to asking "does the collection T contain an empty set?". Naturally, it does. Every collection, even if empty, contains an empty set.

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