Why does type IsNever = T extends never ? true : false give never for IsNever ?

never is an empty union, and distributing over it yields an empty result

It's a compiler bug, so never can't be checked at all

never is always assignable to the literal false in conditional types

TypeScript · Senior · Question 5 of 75

Why does `type IsNever<T> = T extends never ? true : false` give `never` for `IsNever<never>`?

✓Correct answer: never is an empty union, and distributing over it yields an empty result
2It's a compiler bug, so never can't be checked at all
3never is always assignable to the literal false in conditional types

Explanation

A distributive conditional type applies to each union member, and never has no members - the result is never. The correct check is [T] extends [never] ? true : false.

Why does `type IsNever<T> = T extends never ? true : false` give `never` for `IsNever<never>`?

Explanation

Further reading

Problem with a question

Test yourself

Why does type IsNever<T> = T extends never ? true : false give never for IsNever<never>?

Explanation

Further reading

Test yourself

Why does `type IsNever<T> = T extends never ? true : false` give `never` for `IsNever<never>`?