How to write tests for Function

[ilteoood]

I'm new to Rust, so sorry if this is a silly question.

Let's suppose I have a code like:

use napi::bindgen_prelude::Function;

pub type LengthFunction<'a> = Function<'a, &'a str, f64>;

pub fn get_length(text: &str, length_function: Option<LengthFunction>) -> f64 {
  match length_function {
    Some(func) => func.call(text).unwrap(),
    None => text.chars().count() as f64,
  }
}

How can I write a test in Rust to check if the implementation is working correctly?
Otherwise, I think the other alternative I have is to write a JavaScript test.