wasm-bindgen "cannot find function ... in this scope" in CI

By Daniel Zoghalchali·Latchkey

The Rust compiler could not resolve a function the code calls. With wasm-bindgen this usually means an imported JS function is invoked outside its #[wasm_bindgen] extern block, or a feature gating that block is off.

What this error means

A wasm build fails with "error[E0425]: cannot find function alert in this scope" or similar, pointing at a call to a function that should be supplied by JavaScript.

error[E0425]: cannot find function `alert` in this scope
  --> src/lib.rs:10:5
   |
10 |     alert("hello");
   |     ^^^^^ not found in this scope

Diagnose it: which runtime is actually on PATH?

Runners ship multiple versions of most runtimes and select one through a version manager. When a setup action and a version file disagree, the resulting error is about your code rather than the version.

which -a <runtime>
<runtime> --version
echo "PATH=$PATH" | tr ":" "\n" | head -20

# does a version file in the repo disagree with the workflow?
cat .tool-versions .nvmrc .ruby-version .python-version 2>/dev/null

Common causes

The JS import is not declared in an extern block

wasm-bindgen only creates a binding for functions declared inside #[wasm_bindgen] extern "C" { ... }. Calling one that is not declared leaves the name unresolved.

A cfg or feature hides the extern block

The extern block is behind a feature or cfg that is disabled in the CI build, so the binding is not generated.

How to fix it

Declare the import in a wasm-bindgen extern block

1. Add the JS function to a #[wasm_bindgen] extern "C" block.
2. Bring it into scope with use where it is called.
3. Rebuild for the wasm target.

use wasm_bindgen::prelude::*;

#[wasm_bindgen]
extern "C" {
    fn alert(s: &str);
}

Enable the feature that gates the block

If the extern block is feature-gated, pass that feature in CI so the binding compiles.

cargo build --target wasm32-unknown-unknown --features web

How to prevent it

• Declare every JS import inside a #[wasm_bindgen] extern block.
• Avoid hiding bindings behind features that CI does not enable.
• Compile the wasm target in CI so missing bindings fail fast.

Frequently asked questions

Related guides

References

• GitHub Actions documentation