This is the unpublished documentation of wasm-bindgen, the published documentation is available on the main Rust and WebAssembly documentation site . Features documented here may not be available in released versions of wasm-bindgen.

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.

To write an asynchronous test:

  1. Change #[wasm_bindgen_test] into #[wasm_bindgen_test(async)]

  2. Change the return type of the test function to impl Future<Item = (), Error = JsValue>

The test will pass if the future resolves without panicking or returning an error, and otherwise the test will fail.


# #![allow(unused_variables)]
#fn main() {
extern crate futures;
extern crate js_sys;
extern crate wasm_bindgen_futures;

use futures::Future;
use wasm_bindgen::prelude::*;
use wasm_bindgen_futures::JsFuture;

fn my_async_test() -> impl Future<Item = (), Error = JsValue> {
    // 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.
        .map(|x| {
            assert_eq!(x, 42);
        .map_err(|_| unreachable!())