Writing Asynchronous Tests - The `wasm-bindgen` Guide

# 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.

    #![allow(unused)]
    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);
    }
    }

## 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!