Rust wasm32 Build Failures - Native Deps & getrandom in CI

A build for a wasm32 target failed because a dependency relies on native code or OS facilities that don’t exist in wasm - the classic being getrandom having no entropy source on wasm32-unknown-unknown without the js backend enabled.

What this error means

A --target wasm32-unknown-unknown (or wasm32-wasi) build fails with a compile error from a dependency about an unsupported target, missing OS APIs, or getrandom not being supported. The same crate builds fine for a native target.

cargo output

error: the wasm32-unknown-unknown targets are not supported by default; you may
need to enable the "js" feature. For more information see:
https://docs.rs/getrandom/#webassembly-support
   --> getrandom/src/lib.rs

Common causes

A dependency needs a wasm-specific backend

getrandom (pulled in by rand, uuid, etc.) has no entropy source on bare wasm32-unknown-unknown. It needs the js feature (browser) or a custom backend, or the build fails.

Native-only code in the dependency graph

A crate using C bindings, threads, filesystem, or other OS facilities not available on wasm can’t compile for the target unless those parts are feature-gated off.

How to fix it

Enable the wasm backend for getrandom

For browser-targeted wasm, turn on the js feature so getrandom has an entropy source.

Cargo.toml

[target.'cfg(target_arch = "wasm32")'.dependencies]
getrandom = { version = "0.2", features = ["js"] }

Gate native dependencies off for wasm

Use a cfg(not(target_arch = "wasm32")) target table for native-only crates.
Disable default features that pull in OS/native code on the wasm target.
Use wasm32-wasi instead of unknown-unknown when you need a fuller std/OS surface.

How to prevent it

Configure getrandom/RNG backends per the wasm target before building.
Target-gate native-only dependencies with cfg(target_arch = ...).
Build the wasm target in CI so unsupported deps surface on every PR.