WebAssembly New Added 26 September 2026

WebAssembly: Import #0 module="env" error: module is not an object or function

The module declares imports and the importObject handed to instantiate has no property for that namespace, so the engine gives up before linking anything. It is the normal outcome of loading an Emscripten or WASI build with a bare instantiate call, because those toolchains expect their own generated glue to supply env or wasi_snapshot_preview1.

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

// Ask the module what it wants before guessing
const mod = await WebAssembly.compileStreaming(fetch('app.wasm'));
console.table(WebAssembly.Module.imports(mod));

// Supply every namespace it lists, memory and tables included
const { instance } = await WebAssembly.instantiate(mod, {
  env: {
    memory: new WebAssembly.Memory({ initial: 256 }),
    abort: () => { throw new Error('abort'); },
  },
});

// Emscripten output: load the generated .js, do not instantiate the .wasm
// <script src="app.js"></script>   then Module.onRuntimeInitialized

// wasm-bindgen: use the generated init function from pkg/
import init, { greet } from './pkg/app.js';
await init();

// A freestanding build has no imports to satisfy at all
// clang --target=wasm32 -nostdlib -Wl,--no-entry -o app.wasm app.c

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.

developer.mozilla.org emscripten.org

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