Optional chaining should always include `undefined` in its resolved type

TypeScript Version: 3.7.5

Search Terms: optional chaining return type

Code

I have a formatting function, where the input should always be a string, but in certain cases, from the backend, it's undefined for some reason. So to make sure the front-end at least doesn't crash when that happens, I added optional chaining, which fixed the issue.

But the types are kind of off now, and I didn't realize until now because Typescript didn't complain about anything.

export const formatAccountNumber = (account: string): string =>
  account?.replace(/^(\d{4})(\d{2})(\d{5})/, '$1 $2 $3');

Expected behavior: Should get an error/warning about something here. Preferably, I guess optional chaining should always expand the type to include undefined to whatever it is added to? Then, in this example, Typescript could say that the return type is string, but I'm trying to return string | undefined? Either that, or maybe it shouldn't be possible to use optional chaining on a value that does not include either the type undefined and/or null?

Actual behavior: No warnings or errors.

Related Issues: no

まず、optional chaining と明示的な string 戻り値型を使用する TypeScript 3.7.5 の例を再現します。意図した修正が、解決された型に undefined を含めることなのか、null 非許容値に対する optional chaining を拒否することなのかを判断します。選択した動作が一貫して適用され、コンパイラテストでカバーされれば完了です。