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.

try to flip condiction as suggested here github.com/Microsoft/TypeScript/issues/29627 — raffaeleambrosio
– raffaeleambrosio, Commented Jan 7, 2020 at 10:48

phry · Accepted Answer · 2020-01-07 10:45:45Z

11

You can switch the left and right side of the extends, so

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

should work for you.

answered Jan 7, 2020 at 10:45

phry

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3 Comments

Marc Over a year ago

No, it does not work as IsNullable<number> returns true. However if I change from IsBoolean<T> = boolean extends T ? true : false then it works. It seems everything extends null?

Aleksey L. Over a year ago

@Marc, it does work. Make sure strictNullChecks enabled. Playground

phry Over a year ago

Yes, @AlekseyL. has a good point there. If you do not enable strictNullChecks in your tsconfig.json, every type extends null. You should really set strict: null there.

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