napi-rs/examples/napi/src/callback.rs

use napi::bindgen_prelude::*;
use std::env;

#[napi]
fn get_cwd<T: Fn(String) -> Result<()>>(callback: T) {
  callback(env::current_dir().unwrap().to_string_lossy().to_string()).unwrap();
}

/// napi = { version = 2, features = ["serde-json"] }
#[napi]
fn read_file<T: Fn(Result<()>, Option<String>) -> Result<()>>(callback: T) {
  match read_file_content() {
    Ok(s) => callback(Ok(()), Some(s)),
    Err(e) => callback(Err(e), None),
  }
  .unwrap();
}

fn read_file_content() -> Result<String> {
  // serde_json::from_str(&s)?;
  Ok("hello world".to_string())
}