Use ES2015 for mjs build

[shakiba]

Since stage.js migration to ES classes, testbed mjs build is broken, this fixes the issue.

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
planck-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Sep 30, 2024 2:53pm

[zOadT]

Hey! Wouldn't it potentially be too much of a breaking change to already disable the factory functions for es builds?

If I understand the problem correctly, the reason for this error is, that the stage.js code gets bundled as ES6 code (using class-syntax because it doesn't get compiled by TS) and the super() call in WorldStageNode gets down compiled to ES5 syntax (therefore trying to call the Stage.Node constructor without new keyword.
Are you sure this problem only concerns .mjs files? I would expect this to apply to all testbed builds, but maybe I understand the problem wrong here.

Couldn't we also let the stage-js code get compiled to ES5 (either using the typescript plugin or babel)? I think I would prefer this at least for now as it would not change the behavior otherwise. To test it I also tried to move WorldStageNode into a separate JS file (so it does not get compiled to ES5) and I think it also worked (though I would much prefer to compile the stage-js code)

[shakiba]

Alright, let's postpone until another major update; I updated stage.js build to es5 for now.