Let's pretend that life-before-main exists for Rust targeting WebAssembly.

Installation

Add a dependency on wasm-init. This crate intentionally provides a no-op implementation for non-wasm platforms, so cfg handling isn't necessary and we don't pull in unused extra deps.

[dependencies]
wasm-init = "0.2"

User Code

Add as many calls to wasm_init! as required on the Rust side, e.g. for decentralised plugin registration or collecting data from all types that use a particular derive macro.

wasm_init::wasm_init! {
	// literally any code you want to run at startup goes here
}

Initialisation

To ensure your wasm_init! calls are executed on startup, you have three options. Note that all of these options are idempotent - multiple invocations are perfectly safe (if unnecessary), even in a threaded context.

Auto-init feature

If you'd like to skip manually initialising wasm-init, you can enable the auto-init feature as part of the dependency:

[dependencies]
wasm-init = { version = "0.2", features = [ "auto-init" ] }

But please bear in mind that this will prevent you from using a wasm bindgen start function elsewhere in your project.

From Rust

Call the wasm_init::wasm_init function inside your Rust entrypoint.

From JavaScript/TypeScript

Call the wasm_init export from your built module.

<script type="module">
	import init, { wasm_init } from "./pkg/my_wasm_crate.js";
	init().then(() => {
		wasm_init();
		// now do things as normal!
	});
</script>

Why?

inventory et al are cool, but can't yet be used when targeting wasm32-unknown-unknown or similar.