wasm-bindgen-test 0.3.34

Internal testing crate for wasm-bindgen


Read the "Testing with wasm-bindgen-test" section of the guide!


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 temporary 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