wasm-bindgen-test 0.2.21

Internal testing crate for wasm-bindgen
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This crate is an experimental test harness for wasm32-unknown-unknown, with the goal of allowing you to write tests as you normally do in Rust and then simply:

cargo test --target wasm32-unknown-unknown

This project is still in the early stages of its development so there's not a ton of documentation just yet, but a taste of how it works is:

  • First, install the test runner.

    cargo install --path crates/cli

    (this comes with the normal wasm-bindgen CLI tool

  • Next, add this to your .cargo/config:

    runner = 'wasm-bindgen-test-runner'
  • Next, configure your project's dev-dependencies:

    # or [target.'cfg(target_arch = "wasm32")'.dev-dependencies]
    wasm-bindgen-test = { git = 'https://github.com/rustwasm/wasm-bindgen' }
  • Next, write some tests!

    // in tests/wasm.rs
    extern crate wasm_bindgen_test;
    use wasm_bindgen_test::*;
    fn pass() {
        assert_eq!(1, 1);
    fn fail() {
        assert_eq!(1, 2);
  • And finally, execute your tests:

    $ cargo test --target wasm32-unknown-unknown
        Finished dev [unoptimized + debuginfo] target(s) in 0.11s
         Running /home/.../target/wasm32-unknown-unknown/debug/deps/wasm-4a309ffe6ad80503.wasm
    running 2 tests
    test wasm::pass ... ok
    test wasm::fail ... FAILED
    ---- wasm::fail output ----
        error output:
            panicked at 'assertion failed: `(left == right)`
              left: `1`,
             right: `2`', crates/test/tests/wasm.rs:14:5
        JS exception that was thrown:
            RuntimeError: unreachable
                at __rust_start_panic (wasm-function[1362]:33)
                at rust_panic (wasm-function[1357]:30)
                at std::panicking::rust_panic_with_hook::h56e5e464b0e7fc22 (wasm-function[1352]:444)
                at std::panicking::continue_panic_fmt::had70ba48785b9a8f (wasm-function[1350]:122)
                at std::panicking::begin_panic_fmt::h991e7d1ca9bf9c0c (wasm-function[1351]:95)
                at wasm::fail::ha4c23c69dfa0eea9 (wasm-function[88]:477)
                at core::ops::function::FnOnce::call_once::h633718dad359559a (wasm-function[21]:22)
                at wasm_bindgen_test::__rt::Context::execute::h2f669104986475eb (wasm-function[13]:291)
                at __wbg_test_fail_1 (wasm-function[87]:57)
                at module.exports.__wbg_apply_2ba774592c5223a7 (/home/alex/code/wasm-bindgen/target/wasm32-unknown-unknown/wbg-tmp/wasm-4a309ffe6ad80503.js:61:66)
    test result: FAILED. 1 passed; 1 failed; 0 ignored
    error: test failed, to rerun pass '--test wasm'

And that's it! You've now got a test harness executing native wasm code inside of Node.js and you can use cargo test as you normally would for workflows.

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 crate:

fn my_test() -> 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.

This support is currently powered by the wasm-bindgen-futures crate.

Running Tests in Headless Browsers

Add this to the root of your test crate:


Configuring Which Browser is Used

If one of the following environment variables is set, then the corresponding WebDriver and browser will be used. If none of these environment variables are set, then the $PATH is searched for a suitable WebDriver implementation.


Use Firefox for headless browser testing, and geckodriver as its WebDriver.

The firefox binary must be on your $PATH.

Get geckodriver here


Use Chrome for headless browser testing, and chromedriver as its WebDriver.

The chrome binary must be on your $PATH.

Get chromedriver here


Use Safari for headless browser testing, and safaridriver as its WebDriver.

This is installed by default on Mac OS. It should be able to find your Safari installation by default.

Debugging Headless Browser Tests

Set the NO_HEADLESS=1 environment variable and the browser tests will not run headless. Instead, the tests will start a local server that you can visit in your Web browser of choices, and headless testing should not be used. You can then use your browser's devtools to debug.


The test harness is made of three separate components, but you typically don't have to worry about most of them. They're documented here for documentation purposes!


This crate, living at crates/test-macro, is a procedural macro that defines the #[wasm_bindgen_test] macro. The normal #[test] cannot be used and will not work. Eventually it's intended that the #[wasm_bindgen_test] attribute could gain arguments like "run in a browser" or something like a minimum Node version.

For now though the macro is pretty simple and reexported from the next crate, wasm-bindgen-test.


This is the runtime support needed to execute tests. This is basically the same thing as the test crate in the Rust repository, and one day it will likely use the test crate itself! For now though it's a minimal reimplementation that provides the support for:

  • Printing what test cases are running
  • Collecting console.log and console.error output of each test case for printing later
  • Rendering the failure output of each test case
  • Catching JS exceptions so tests can continue to run after a test fails
  • Driving execution of all tests

This is the crate which you actually link to in your wasm test and through which you import the #[wasm_bindgen_test] macro. Otherwise this crate provides a console_log! macro that's a utility like println! only using console.log.

This crate may grow more functionality in the future, but for now it's somewhat bare bones!


This is where the secret sauce comes into play. We configured Cargo to execute this binary instead of directly executing the *.wasm file (which Cargo would otherwise try to do). This means that whenever a test is executed it executes this binary with the wasm file as an argument, allowing it to take full control over the test process!

The test runner is currently pretty simple, executing a few steps:

  • First, it runs the equivalent of wasm-bindgen. This'll generate wasm-bindgen output in a temoprary directory.
  • Next, it generates a small shim JS file which imports these wasm-bindgen-generated files and executes the test harness.
  • Finally, it executes node over the generated JS file, executing all of your tests.

In essence what happens is that this test runner automatically executes wasm-bindgen and then uses Node to actually execute the wasm file, meaning that your wasm code currently runs in a Node environment.

Future Work

Things that'd be awesome to support in the future:

  • Arguments to wasm-bindgen-test-runner which are the same as wasm-bindgen, for example --debug to affect the generated output.
  • Running each test in its own wasm instance to avoid poisoning the environment on panic