Conditional types for nullable in Typescript

Question

I want to check if a type is nullable or not, and if it has a conditional type on the value.

I tried implementing

type IsNullable<T> = T extends null ? true : false;

However, it does not seem to work

type test = IsNullable<number> // Returns false as it should
type test = IsNullable<number | null> // Returns false when it should be true

What's the proper way of checking if a type is nullable? I tried with T extends null | T and did not work either.

Answer 1

您可以切换 的左侧和右侧extends，因此

type IsNullable<T> = null extends T ? true : false;

应该为你工作。