为什么 Typescript 允许在 function 中为空/未定义

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

Answer 1

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),