Writing Asynchronous Tests

Not all tests can execute immediately and some may need to do “blocking” work like fetching resources and/or other bits and pieces. To accommodate this asynchronous tests are also supported through the futures and wasm-bindgen-futures crates.

Writing an asynchronous test is pretty simple, just use an async function! js_sys::Promise implements IntoFuture, so you can .await it directly:

#![allow(unused)]
fn main() {
use wasm_bindgen::prelude::*;

#[wasm_bindgen_test]
async fn my_async_test() {
    // Create a promise that is ready on the next tick of the micro task queue.
    let promise = js_sys::Promise::resolve(&JsValue::from(42));

    // Await the promise directly — no conversion wrapper needed.
    let x = promise.await.unwrap();
    assert_eq!(x, 42);
}
}

If you need a named Future value, JsFuture is available from js_sys::futures or from wasm_bindgen_futures:

#![allow(unused)]
fn main() {
use wasm_bindgen_futures::JsFuture;

let x = JsFuture::from(promise).await.unwrap();
}

Rust compiler compatibility

Note that async functions are only supported in stable from Rust 1.39.0 and beyond.

If you’re using the futures crate from crates.io in its 0.1 version then you’ll want to use the 0.3.* version of wasm-bindgen-futures and the 0.2.8 version of wasm-bindgen-test. In those modes you’ll also need to use #[wasm_bindgen_test(async)] instead of using an async function. In general we’d recommend using the nightly version with async since the user experience is much improved!