asynchronous-tests.md

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

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);
}

Rust compiler compatibility

Note that async functions are only supported in stable from Rust 1.39.0 and beyond. As of the time of this writing (2019-09-05) this is the Nightly channel of Rust.

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!