[dev-dependencies]
wasm-bindgen-test = "0.3.0"

Note that the 0.3.0 track of wasm-bindgen-test supports Rust 1.39.0+, which is currently the nightly channel (as of 2019-09-05). If you want support for older compilers use the 0.2.* track of wasm-bindgen-test.

Create a $MY_CRATE/tests/wasm.rs file:

# #![allow(unused_variables)]
#fn main() {
use wasm_bindgen_test::*;

#[wasm_bindgen_test]
fn pass() {
    assert_eq!(1, 1);
}

#[wasm_bindgen_test]
fn fail() {
    assert_eq!(1, 2);
}

// On a target other then `wasm32-unknown-unknown`, the `#[test]` attribute
// will be used instead, allowing this test to run on any target.
#[wasm_bindgen_test(unsupported = test)]
fn all_targets() {
    assert_eq!(1, 2);
}
#}

Writing tests is the same as normal Rust #[test]s, except we are using the #[wasm_bindgen_test] attribute.

One other difference is that the tests must be in the root of the crate, or within a pub mod. Putting them inside a private module will not work.

Run the tests with wasm-pack test. By default, the tests are generated to target Node.js, but you can configure tests to run inside headless browsers as well.

$ wasm-pack test --node
    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'

That's it!

⚠️ The recommended way to use wasm-bindgen-test is with wasm-pack, since it will handle installing the test runner, installing a WebDriver client for your browser, and informing cargo how to use the custom test runner. However, you can also manage those tasks yourself, if you wish.

In addition to the steps above, you must also do the following.

The test runner comes along with the main wasm-bindgen CLI tool. Make sure to replace "X.Y.Z" with the same version of wasm-bindgen that you already have in Cargo.toml!

cargo install wasm-bindgen-cli --vers "X.Y.Z"

Add this to $MY_CRATE/.cargo/config:

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

Run the tests by passing --target wasm32-unknown-unknown to cargo test:

cargo test --target wasm32-unknown-unknown