Never use JavaScript on the server side. The amount of bugs that can happen is insane. JavaScript is just a specification with varying implementations across various versions.
A proper language like Java, Go, or Rust ensures that your code will have fewer logical bugs of this type.
I also support using whatever language you and your team prefer when you can. that's the glory of backend: no restrictions on what you can run. but sometimes you need to write client software, and those are strictly easier to manage in the platform's native tongue: Swift, Kotlin, JS, and so on
The function is unambiguously written, but the runtime functions differently, and this is not a language problem? This is incoherent; one of these statements is incorrect.
The language absolutely did something wrong, by trying to evaluate a non-boolean type as a boolean. That is a horrible footgun and JS is absolutely at fault for doing so.
Can JS succeed in the back end? I say yes, but not like this.