In Javascript, the concept of an abstract class is not natively supported as it is in other languages such as Java, TypeScript, and Python. However...
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Usually an abstract class is not meant to be instatiated, but it does not necessarily need to be protected against instatiation. If you define a class in an ES6-moduel, simply do not expose the class name. or choose a name that will not be used by accident. The risk you use such a class will be very low.