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! You'll also likely want to use the wasm-bindgen-futures crate to convert JS promises to Rust futures.

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

#[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));

    // Convert that promise into a future and make the test wait on it.
    let x = JsFuture::from(promise).await.unwrap();
    assert_eq!(x, 42);
}
#}

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!