No. 248 — Is it never? — Annotated solution

Is it never?

Implement IsNever<T> so it is true when T is exactly never, and false for everything else.

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Par 2

Puzzle

is-never.ts

type IsNever<T> = ???

type IsNever<T> = ???

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Replace ??? — your solution is checked against the cases below. Tab indents; press Escape then Tab to move focus out.

Checks

IsNever<never>—
→ true
IsNever<string>—
→ false
IsNever<never | string>—
→ false

How a check is judged Exact type equality, not assignability — an intersection is not the same as the flattened object.

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Annotated solution

Published September 1, 2026

The solution

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

The common wrong answer

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

This returns never, not true. A conditional type with a naked type parameter distributes over unions, and never is the empty union — so there are no members to distribute over, the conditional never runs for any member, and the result is the empty union again.

Line by line

[T] extends [never]
The tuple wrapper is not decoration: it makes the left side no longer a naked type parameter, which switches distribution off and lets the comparison happen on never itself.
IsNever<never | string>
never | string is just string — never vanishes from a union the moment it joins one. The answer is false because the type that arrived was never never to begin with.

Takeaway

never is the empty union, and that single fact explains most of its strange behaviour: distribution over it does nothing, and it disappears from any union it joins.