Module imports without extension fail loading

I’ve configured regarding the documentation as follows

karma.config.js

    files: [
      { pattern: 'test/**/*.js', type: 'module' },
      { pattern: 'src/**/*.js', type: 'module', included: false }
    ],

Then when using a working mocha test with imports without .js extensions, like

import TestModule from '../../src/TestModule';

…the execution with karma fails with:

...
26 05 2021 15:09:57.267:WARN [web-server]: 404: /base/src/TestModule
Chrome Headless 90.0.4430.212 (Mac OS 10.15.7) ERROR
  Uncaught Error loading test/Test.js
  at http://localhost:9876/context.html:286:271

Importing ‘…/…/src/TestModule.js’ does work, but is not a solution as the TestModule.js itself imports without extensions. The implementation of TestModule.js shall not be changed.

Is this a bug in Karma runner or did a missed an important configuration here? Help appreciated

[devoto13]

Seems to work as intended: https://stackoverflow.com/a/55289496/1377864

In a browser with support for ES Modules (ESM), no extensions will be added to the URL that you import - if your file for example has .js extension, you have to write import {x} from “./file.js”.

Unless you told it to do so, Karma will not do any file processing, so it will just load the file “as is” in the browser.

[devoto13]

No, there is no way to do it without plugins. Vanilla Karma just loads your JS code in the browser without any extra processing and therefore JS should be in the format which the browser understands.