wasm-bindgen/crates/test/README.md

wasm-bindgen-test

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:

  [target.wasm32-unknown-unknown]
  runner = 'wasm-bindgen-test-runner'
  

• Next, configure your project's dev-dependencies:

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

  WARNING: the console_error_panic_hook has a dependency on wasm-bindgen from crates.io which conflicts with the one from git used by wasm-bindgen-test: it produces linker errors due to duplicated symbols.

  Until wasm-bindgen-test is released on crates.io, the temporary workaround is to patch the crates.io's wasm-bindgen dependency to be the same that wasm-bindgen-test uses by adding the following to your project's Cargo.toml:

  [patch.crates-io]
  wasm-bindgen = { git = 'https://github.com/rustwasm/wasm-bindgen' }
  

• Next, write some tests!

  // in tests/wasm.rs
  #![feature(use_extern_macros)]
  
  extern crate wasm_bindgen_test;
  
  use wasm_bindgen_test::*;
  
  #[wasm_bindgen_test]
  fn pass() {
      assert_eq!(1, 1);
  }
  
  #[wasm_bindgen_test]
  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
  
  failures:
  
  ---- 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)
  
  
  failures:
  
      wasm::fail
  
  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:

#[wasm_bindgen_test(async)]
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.

Components

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!

wasm-bindgen-test-macro

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.

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!

wasm-bindgen-test-runner

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