MV3: How to properly export common.js to background.js script?

Add-ons Development
#1

I am migrating my browser extension to Manifest V3. My background.js and content script reuse a common JS function.

I can’t find any way to import/export that function into background.js, as detailed in my question on StackExchange (ready to duplicate its contents here if required).

What am I doing wrong?

In Chrome MV3, the following code just works fine:

common.js:

export function commonFunction()
{
	return 1;
}

background.js:

import { commonFunction } from './common.js';

manifest.json:

  ...
  "background": {
    "service_worker": "background.js",
    "type": "module"
  },

  "web_accessible_resources": [
    {
      "resources": ["common.js"],
      "matches": ["https://*.example.com/*"]
    }
  ]
  ...
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