Why Typescript allows null/undefined in function

Question

Why Typescript allows to pass null / undefined there?

// "strictNullChecks": false
function someFun(param: (foo: any) => any) {}
someFun(null); // no error - incorrect
someFun(undefined); // no error - incorrect

It allows it because, as you know, you have --strictNullChecks disabled. Are you asking "why does disabling --strictNullChecks disable strict null checks"? That's answered below. Or are you asking "why is --strictNullChecks disabled by default"? That's probably answered here: it would break existing real world code that used TypeScript before these --strict options existed. — jcalz
– jcalz, Commented Oct 24, 2019 at 14:48

Community · Accepted Answer · 2020-06-20 09:12:55Z

2

Quoting from the docs

In strict null checking mode, the null and undefined values are not in the domain of every type and are only assignable to themselves and any (the one exception being that undefined is also assignable to void)

..T and T | undefined are considered synonymous in regular type checking mode (because undefined is considered a subtype of any T),

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Oct 24, 2019 at 9:51

jperl

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