[es-dev-server] Default babel configuration breaks custom one

[Lodin]

I'm using es-dev-server through the karma-esm, lerna, and united custom karma configuration for each package of multirepository. To start karma in a separate package, I need to import babel.config.json in the karma configuration file and load it through the babelConfig of karma-esm.

However, when I do it, karma fails with the following error:

Error compiling: Duplicate plugin/preset detected.

My babel.config.json file is the following:

{
  "comments": false,
  "presets": ["@babel/preset-typescript"],
  "plugins": [
    [
      "@babel/plugin-proposal-nullish-coalescing-operator",
      {
        "loose": true
      }
    ],
    ["@babel/plugin-proposal-optional-chaining", {"loose": true}]
  ]
}

I just require it as a JSON file and push it to babelConfig option.

After some research, I found that es-dev-server loads some default configuration that includes @babel/plugin-proposal-nullish-coalescing-operator, @babel/plugin-proposal-optional-chaining, etc. That configuration merges with my configuration and due to name differences has the duplicates inside. It happens here.

I think there should be some checks that disables default configuration if the user configuration is used or a some kind of option to disable default configuration.

[Lodin]

UPDATE: Issue happens only with @babel/plugin-proposal-nullish-coalescing-operator, @babel/plugin-proposal-optional-chaining on board. There is a remark that these plugins could be removed when babel/babel#10811 is merged. It is merged 21 days ago, the latest babel release was 7.8.5, so I think it can be removed now. I can create a PR for it if you don't mind.

However, still something should be done for deduping plugins or at least the list of used plugins should be mentioned in the documentation somewhere.

[LarsDenBakker]

There is a PR for removing those plugins #1242

I'm surprised this causes babel conflicts, I wasn't aware of that. That makes it pretty hard to compose plugins. Will look into it

[Lodin]

BTW, the workaround for this issue is quite simple. You just need to make sure your plugins that conflict with the default ones are moved to a separate environment and not affect the test one.