Using getrandom with js feature and wasm32-unknown-unknown breaks wasm library


I'm compiling a set of libraries as dependencies to a WASM library, using the "raw" wasm32-unknown-unknown target ("raw" in the sense that I don't use wasm-bindgen or wasm-pack or anything).

That works great until one of the libraries needs the rand crate, or more specifically the getrandom crate. I already found out that I need to enable the js feature for getrandom, but as soon as I do that, I can't instantiate my WebAssembly library anymore.

The error message I get in the console is:

TypeError: WebAssembly.instantiate(): Imports argument must be present and must be an object

It seems like activating this feature leads to wasm-bindgen being used in some form or another ? Does that mean that I can only use it in conjuction with wasm-bindgen, not with a "raw" library ?


You can not get randomness on the "raw" target without additional information about enviroment. The js feature assumes that you use wasm-bingen and it's one of the reasons why it's disabled by default. If you don't want to rely on wasm-bingen, then you should use the custom feature to provide custom randomness source specific to your environment.

Oooh ok, thanks for the hint ! I saw that the documentation mentions the wasm-bindgen toolchain, but I wasn't aware that it's exclusive to that toolchain ...

I run into this issue all the time at work where upstream dependencies think the only way to run WebAssembly is via a JavaScript environments.

It means you need to make a PR to that upstream library so it exposes a js feature which will enable getrandom/js in turn.

At a previous company, we would implement getrandom's custom handler by importing some sort of of fn gen_random(buffer: &mut [u8]) function from the host so the application could fill your buffer with randomness (example).

Such approach is not recommended. getrandom docs are pretty clear about it:

This feature should only be enabled for binary, test, or benchmark crates. Library crates should generally not enable this feature, leaving such a decision to users of their library. Also, libraries should not introduce their own js features just to enable getrandom’s js feature.

getrandom goes to a great length to support non-JS WASM applications.

1 Like