Inject Env

Inject Env

The #[napi] macro is a very high level abstraction for the Node-API. Most of the time, you use the Rust native API and crates.

But sometimes you still need to access the low-level Node-API, for example, to call napi_async_cleanup_hook (opens in a new tab) or napi_adjust_external_memory (opens in a new tab).

For this scenario, NAPI-RS allows you to inject Env into your fn which is decorated by the #[napi].

use napi::{Env, bindgen_prelude::*};
fn call_env(env: Env, length: u32) -> Result<External<Vec<u32>>> {
  env.adjust_external_memory(length as i64)?;
  Ok(External::new(vec![0; length as usize]))

And the Env will be auto injected by NAPI-RS, it does not affect the arguments types in the JavaScript side:

export function callEnv(length: number) -> ExternalObject<number[]>

You can also inject Env in impl block:

use napi::bindgen_prelude::*;
// A complex struct which can not be exposed into JavaScript directly.
struct QueryEngine {}
#[napi(js_name = "QueryEngine")]
struct JsQueryEngine {
  engine: QueryEngine,
impl JsQueryEngine {
  pub fn with_initial_count(count: u32) -> Self {
    JsQueryEngine { engine: QueryEngine::with_initial_count(count) }
  /// Class method
  pub fn query(&self, env: Env, query: String) -> napi::Result<String> {
    self.engine.query(query).map_err(|err| Error::new(Status::GenericFailure, format!("Query failed {}", err)))

The behavior is just the same with the pure fn.