Null coalescing operator for "void" produces "void | null" type

[adamplonka]

TypeScript Version: 3.7.x-dev.201xxxxx

Search Terms: null coalesce void, void null, void is not assignable

Code

function a(): void | string {
}

function b(): null | string {
   return a() ?? null;
}

Expected behavior:
Should compile with no errors, resulting type is "null | string"

Actual behavior:
Error:

Type 'string | void | null' is not assignable to type 'string | null'.
  Type 'void' is not assignable to type 'string | null'.(2322)

Resulting type is "null | void | string"

Playground Link: Playground Link

Related Issues: didn't find any related issues

[MartinJohns]

Duplicate (or at least heavily related): #35236 / #35850.

A void return type does not mean "nothing". It means "anything, but you shouldn't use this for anything". It could be a undefined, but it could just as well be a string. It can be truthy or falsy.

https://github.com/Microsoft/TypeScript/wiki/FAQ#why-are-functions-returning-non-void-assignable-to-function-returning-void

---

IMO this should actually error due to #26262.

[adamplonka]

This is the javascript code produced by the typescript code above

var _a;
var b = (_a = a()) !== null && _a !== void 0 ? _a : null;

When you use this javascript back as typescript the b will be of type string | null and not string | null | void
Shouldn't typescript be consistent with resulting javascript?

[RyanCavanaugh]

It's legal for a to (indirectly) return 0, for example, which would cause b to return 0 which is neither null or string.

See also https://stackoverflow.com/questions/58885485/why-does-typescript-have-both-void-and-undefined/58885486

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.