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.

Try the puzzle yourself

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.