[benatkin]

Because this is an internal function in a _library_, and you should be willing to look inside it if you're changing a call to it.

Here's where whether or not it's defined can be traced to: https://github.com/hotwired/turbo/blob/41c074ff113a8882aadbc...

Maybe the types should have been written differently, to indicate that it's the type of the submitter property in a FormEvent. That's another cost of using TypeScript, is trying to get the types right. TMTOWTDI.

[seanwilson]

> Because this is an internal function in a _library_

Why does that matter? Error-prone approaches create buggy code and we don't want buggy code, whether it's a library or something else. Libraries can be huge and complex, and arguably should have more robust edge case checking than regular code.