WebAssembly New Added 19 September 2026

WebAssembly: LinkError: import object field is not a Function

Instantiation failed because the import object handed to the module does not match what the module declares it needs: a missing property, a nested namespace that is undefined, or a value where a function was expected. The module name comes first in the error, so env.abort means the module asked for abort inside the env namespace.

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

// The shape of the object has to mirror the module imports exactly
const imports = {
  env: {
    abort: (msg, file, line, col) => { throw new Error("wasm abort " + line); },
    memory: new WebAssembly.Memory({ initial: 256 }),
  },
};
const { instance } = await WebAssembly.instantiateStreaming(fetch("app.wasm"), imports);

# List what the module actually wants before guessing
wasm-objdump -x app.wasm | sed -n '/Import/,/Export/p'

// The same list from JavaScript, without the toolchain
WebAssembly.Module.imports(await WebAssembly.compileStreaming(fetch("app.wasm")))

// AssemblyScript and Emscripten generate a loader that builds this object for you:
// importing the raw .wasm and hand rolling the imports is the usual cause
// import { instantiate } from "./build/release.js";

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 webassembly.github.io

Related WebAssembly errors

See all 10 WebAssembly errors →

Browse other categories

Something missing or wrong?

This entry is maintained by hand. If the fix is out of date, incomplete, or you have a better one, email a correction and it will be reviewed.