TypeError: Incorrect response MIME type. Expected 'application/wasm'

[tennisonchan]

Since application/wasm isn't an official MIME type yet, WebAssembly/spec#573, in case you got the type error of an incorrect response MIME type, you can add the following line in file /etc/mime.types and then restart the server.

application/wasm    wasm

FYI:
https://bugs.python.org/issue31670
python/cpython#3861

[timmyjose]

Just wanted to make a note here for macOS users. For me, I was running into the same issue, and the suggestions online (of, say, adding a new MIME type to /etc/mime) do not work for macOS since there is no such file).

The following snippet does not work on macOS:

<script>
    WebAssembly.instantiateStreaming(fetch('my-file.wasm'))
      .then(obj => {
         // do something, say, print to console
         console.log(obj.instance.exports.my_func());
      });
  </script>

whereas the following does work:

<script>
  fetch('my-file.wasm').then(response => 
  	response.arrayBuffer()
  ).then(bytes => 
    WebAssembly.instantiate(bytes)
  ).then(obj => {
      console.log(obj.instance.exports.my_func());
  });
  </script>

Posting it here in case other frustrated people come across this looking for a solution on macOS.

[richlum]

similar issue using 'web server for chrome' on linux. Adapted timmyjose solution while trying to follow along https://developer.mozilla.org/en-US/docs/WebAssembly/Text_format_to_wasm

<script>
  var importObject = {
    imports: { imported_func: arg => console.log(arg) }
  };
  fetch('simple.wasm').
  then(response => response.arrayBuffer()).
  then(bytes => WebAssembly.instantiate(bytes, importObject)).
  then(obj => obj.instance.exports.exported_func());

[lukewagner]

I think python2 -m SimpleHTTPServer and python3 -m http.server should work for local testing nowadays.

Note that instantiateStreaming(Response) provides much better performance than instantiate(ArrayBuffer) so ideally an app would only use instantiate(ArrayBuffer) as a fallback when instantiateStreaming failed. (E.g., I think Unity and Emscripten do this.)

[H-arshit]

<script> fetch('my-file.wasm').then(response => response.arrayBuffer() ).then(bytes => WebAssembly.instantiate(bytes) ).then(obj => { console.log(obj.instance.exports.my_func()); }); </script>

where do we have make these changes?

[lucidBrot]

I think python2 -m SimpleHTTPServer and python3 -m http.server should work for local testing nowadays.

Note that instantiateStreaming(Response) provides much better performance than instantiate(ArrayBuffer) so ideally an app would only use instantiate(ArrayBuffer) as a fallback when instantiateStreaming failed. (E.g., I think Unity and Emscripten do this.)

That did not directly work for me, however this gist provides a python server that does work

[VertikaJain]

You need to install static-server rather than http-server. This is because the WebAssembly module needs to be served with a MIME type of application/wasm, and the MIME type is provided in the static-server package.
For confirmation, you can cross check in your package-lock.json after installing both these packages using npm and compare the respective "requires" fields. The static-server will be having "mime": "^version_name" .
Check out my github repository on AssemblyScript for more detailed implementation.