Promise

`Promise<T>`

Awaiting a JavaScript Promise in Rust sounds crazy, but it's feasible in NAPI-RS. The Promise<T> in NAPI-RS implements the std::future::Future trait, so you can use the await keyword to await it.

💡

Awaiting a JavaScript Promise needs the tokio_rt and napi4 features to be enabled.

Promise<T> is Send, so you can freely use it in the async context.

lib.rs

use napi::bindgen_prelude::*;
 
#[napi]
pub async fn async_plus_100(p: Promise<u32>) -> Result<u32> {
  let v = p.await?;
  Ok(v + 100)
}

test.mjs

import { asyncPlus100 } from './index.js'
 
const fx = 20
const result = await asyncPlus100(
  new Promise((resolve) => {
    setTimeout(() => resolve(fx), 50)
  }),
)
 
console.log(result) // 120

`PromiseRaw<'env, T>`

PromiseRaw<'env, T> represent the raw Promise value in the JavaScript, it contains the lifetime so it can only be used in the sync context.

But conveniently, it can call methods on the JavaScript Promise, such as then, catch, and finally.

lib.rs

use napi::bindgen_prelude::*;
use napi_derive::napi;
 
#[napi]
pub fn promise_callback(promise: PromiseRaw<u32>) -> Result<PromiseRaw<u32>> {
  promise.then(|ctx| Ok(ctx.value + 100))
}

index.ts

import { promiseCallback } from './index.js'
 
const value = await promiseCallback(Promise.resolve(100))
 
console.log(value) // 200

External Types Overwrite