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.
TIP
Awaiting a JavaScript Promise needs the async or tokio_rt feature.
tokio_rt enables napi4 for you.
INFO
Promise<T> is Send when T is Send, so the compiler prevents a
non-Send resolved value from crossing Tokio worker threads.
lib.rs
rust
use napi::bindgen_prelude::*;
use napi_derive::napi;
#[napi]
pub async fn async_plus_100(p: Promise<u32>) -> Result<u32> {
let v = p.await?;
v.checked_add(100)
.ok_or_else(|| Error::new(Status::InvalidArg, "result exceeds u32"))
}
test.mjs
js
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
rust
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
js
import { promiseCallback } from './index.js'
const value = await promiseCallback(Promise.resolve(100))
console.log(value) // 200