WebAssembly InstantiateStreaming Wrong MIME type

Question

I am attempting to get this tutorial (here: https://www.hellorust.com/demos/add/index.html) to work, and it seems that whatever I do, I cannot get the WebAssembly MDN reserved f

Answer 1

there was some error; TypeError: "Response has unsupported MIME type" The Web server you are running does not understands/serves a MIME type application/wasm.

You can use a rust based http server, it knows about wasm MIME type.

Installation

Simply use curl

curl -SsL https://cdn.rawgit.com/thecoshman/http/master/install.sh | sh

and execute the downloaded script or you can explorer other ways to do the same at https://crates.io/crates/https.

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Running

Please use the downloaded server to server your Web Application(index.html). e.g

cd ${YOUR_APPS_PATH}

http

Answer 2

A snippet of code for a workaround has been published on the WebAssembly Git here. Unfortunately, this is a workaround, and this defeats the purpose of instantiateStreaming() which is told here to be "a lot more efficient", since the workaround needs an ArrayBuffer that instantiateStreaming() helps to avoid.

Answer 3

Considering you can't change the server to properly return application/wasm for .wasm file requests for any reason, you can work around the issue by changing the way you instantiate the WebAssembly module. Instead of doing this:

WebAssembly.instantiateStreaming(fetch("./add.wasm")).then(obj => /* ... */)

Do this:

const response = await fetch("add.wasm");
const buffer = await response.arrayBuffer();
const obj = await WebAssembly.instantiate(buffer);
obj.instance.exports.exported_func();

Or the equivalent using then() if you cannot use async/await.

In practice, what my workaround does is to avoid calling instantiateStreaming(), which must check the MIME type returned by the server before proceeding (according to this specification). Instead, I call instantiate() passing an ArrayBuffer and avoid the check altogether.