fix(babel-preset-mc-app): to add loose private methods

[tdeekens]

Summary

We're receiving a lot of warnings internally when building with the recent babel update with about 40k lines of this on CI:

Though the "loose" option was set to "false" in your @babel/preset-env config, it will not be used for @babel/plugin-proposal-private-methods since the "loose" mode option was set to "true" for @babel/plugin-proposal-class-properties.
The "loose" option must be the same for @babel/plugin-proposal-class-properties, @babel/plugin-proposal-private-methods and @babel/plugin-proposal-private-property-in-object (when they are enabled): you can silence this warning by explicitly adding

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🦋 Changeset detected

Latest commit: a440704

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
@commercetools-frontend/babel-preset-mc-app	Patch
@commercetools-frontend/jest-preset-mc-app	Patch
@commercetools-frontend/mc-scripts	Patch

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[emmenko]

Thanks but unfortunately that does not fully work.

Last time I tried to fix that, Cypress has problems: https://github.com/commercetools/merchant-center-application-kit/runs/2509665796

I haven't looked too much into it but apparently something else needs to be fixed/changed as well.
If you have the time, feel free to look into it.

[tdeekens]

For others and me to document:

We use loose on class properties as the otherwise used defineProperty comes with a performance penalty. Currently create-react-app doesn't have and special configuration for private methods.

When true, private methods will be assigned directly on its parent via Object.defineProperty rather than a WeakSet. This results in improved performance and debugging (normal property access vs .get()) at the expense of potentially leaking "privates" via things like Object.getOwnPropertyNames.

So I don't see why Cypress chokes on this as both syntaxes should work. Will investigate further.

Reference: https://babeljs.io/docs/en/babel-plugin-proposal-private-methods

[emmenko]

Good idea!