async fn

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You must enable the async or tokio_rt feature in napi to use async fn:

Cargo.toml

[dependencies]
napi = { version = "2", features = ["async"] }

You can do a lot of async/multi-threaded work with AsyncTask and ThreadsafeFunction, but sometimes you may want to use the crates from the Rust async ecosystem directly.

NAPI-RS supports the tokio runtime by default. If you await a tokio future in async fn, NAPI-RS will execute it in the tokio runtime and convert it into a JavaScript Promise.

lib.rs

use futures::prelude::*;
use napi::bindgen_prelude::*;
use tokio::fs;
 
#[napi]
async fn read_file_async(path: String) -> Result<Buffer> {
  fs::read(path)
    .map(|r| match r {
      Ok(content) => Ok(content.into()),
      Err(e) => Err(Error::new(
        Status::GenericFailure,
        format!("failed to read file, {}", e),
      )),
    })
    .await
}

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index.d.ts

export function readFileAsync(path: string): Promise<Buffer>

Reference External