This is the unpublished documentation of wasm-pack, the published documentation is available on the main Rust and WebAssembly documentation site .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-pack.

src/utils.rs

The purpose of utils.rs is to define the utils module, which contains a single function set_panic_hook. This function becomes part of the utils module in lib.rs, as described in the preceding section.

If the console_error_panic_hook feature is not enabled, then set_panic_hook is defined to be an inlined empty function. So, there is no run-time performance or code-size penalty incurred by its use.

We will discuss:

  1. Defining set_panic_hook
  2. What is console_error_panic_hook?

1. Defining set_panic_hook


#![allow(unused)]
fn main() {
pub fn set_panic_hook() {
    // When the `console_error_panic_hook` feature is enabled, we can call the
    // `set_panic_hook` function at least once during initialization, and then
    // we will get better error messages if our code ever panics.
    //
    // For more details see
    // https://github.com/rustwasm/console_error_panic_hook#readme
    #[cfg(feature = "console_error_panic_hook")]
    console_error_panic_hook::set_once();
}
}

Here, we define a function that's preceded by a cfg attribute. This attribue, #[cfg(feature = "console_error_panic_hook")], tells Rust to check if the console_error_panic_hook feature is set at compile time. If it is, it will call this function. If it isn't- it won't!

2. What is console_error_panic_hook?

The crate console_error_panic_hook allows debugging Rust panic messages in a web browser, making it much easier to debug WebAssembly code.

Let's compare what happens when Rust code panics before and after enabling the feature:

Before: "RuntimeError: Unreachable executed"

After: "panicked at 'index out of bounds: the len is 3 but the index is 4', libcore/slice/mod.rs:2046:10"

To do this, a panic hook is configured that logs panics to the developer console via the JavaScript console.error function.

Note though that console_error_panic_hook is not entirely automatic, so you'll need to make sure that utils::set_panic_hook() is called before any of our code runs (and it's safe to run set_panic_hook many times).

For more details, see the console_error_panic_hook repository.