WebAssembly New Added 19 September 2026

wasm-bindgen: recursive use of an object detected

A Rust method holding a mutable borrow of a struct called into JavaScript, and that JavaScript called back into the same object before the first call returned. Rust borrow rules are enforced at runtime across the boundary, so what would be a compile error inside Rust becomes this exception in the browser.

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

Quick fix

// Typical shape: &mut self calls JS, and the JS handler calls back in
#[wasm_bindgen]
impl App {
    pub fn on_click(&mut self) {
        self.render();          // render() dispatches a DOM event
    }                           // that event handler calls app.on_click() again
}

// Take &self and keep the mutable state behind a RefCell you can drop early
#[wasm_bindgen]
pub struct App { state: std::cell::RefCell<State> }

#[wasm_bindgen]
impl App {
    pub fn on_click(&self) {
        {
            let mut s = self.state.borrow_mut();
            s.count += 1;
        }                        // borrow ends here, before any call into JS
        self.render();
    }
}

// Or defer the re-entrant call so the first one has returned
// queue_microtask or requestAnimationFrame from the JS side

How to diagnose WebAssembly errors

WebAssembly errors are precise about the layer that failed. A CompileError means the bytes are not a valid module: most often the file was served with the wrong MIME type, or an HTML error page was fetched instead of the .wasm file. A RuntimeError: memory access out of bounds means the module read or wrote outside its linear memory, which in a language like C or Rust is a genuine memory-safety bug caught by the sandbox.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

Check what the server actually returned. The wasm file must be served as application/wasm for streaming compilation, and a 404 HTML page produces a misleading magic-number error.
Validate the module offline with wasm-validate and inspect it with wasm-objdump -x from the WABT toolkit.
For out-of-bounds errors, rebuild with sanitizers or debug assertions in the source language. The wasm runtime cannot tell you which source line was responsible without DWARF info.
Confirm imports match: every function the module imports must be supplied by the host with the exact name and signature, or instantiation fails.
Build with debug symbols and use the browser's DWARF support to step through original source rather than raw wasm.

Tools worth reaching for

wasm-validate / wasm-objdump (WABT)
browser DWARF debugging
wasmtime --invoke
curl -I (check MIME type)

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

rustwasm.github.io doc.rust-lang.org

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