msgpack-lite | Fast Pure JavaScript MessagePack Encoder and Decoder / | User Interface library

What is the Rust equivalent of a JavaScript object

That is a HashMap:

The error message tells you you sent the wrong type of object to the decode() method. The buffer decoding documentation shows that only buffers, arrays and Uint8Array objects are supported.

Change the response type to arraybuffer (from the default text), then feed the response data to msgpack.decode() as a Uint8Array typed array:

To set a global variable from inside a module (e.g. index.js), you can add it directly to the window object. Otherwise, the variable will only live in the context of the module.

window.msgpack = require("msgpack-lite");

I think there is another solution where you require("msgpack-lite") inside your native Elm module. This way, you don't need a global variable but instead you have another compilation step for your native Elm module.