simple-web-worker | An utility to simplify the use of web workers | Web Framework library

I am trying to make the web worker example from MDN to work when it is not served from a server but opened directly open from my local filesystem (via a via file:// URL)

The example works fine when served from Apache at http://localhost/1/simple-web-worker-gh-pages/index.html but it doesn't when it is read locally at file:///var/www/html/1/simple-web-worker-gh-pages/index.html

The developer console shows that messages are posted but there is no answer from the worker. (I tried both Firefox and Chromium.)

How can I make it to work ?

(There's a similar question failed to load script - Webworker (PDF.JS) , but my problem is different, as I don't see a "Failed to load script" error.)

Reading about web workers I stumbled across this example from mdn. The worker.js is just once simple function. So when we post a message to the worker the onmessage part is then started by the worker. In the simple example the worker just multiplies two numbers. But what if I want a worker that can also add, divide etc? Do I need to create a new worker file (e.g. worker_add.js) for each function I want my worker to run? Or what is the cleaner way to deal with this?

I was thinking of having a string posted as first argument to the worker