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  • Karl Giesing thinks this is interesting:

class syntax does not provide a way to declare class member properties (only methods). So if you need to do that to track shared state among instances

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Cover of You Don't Know JS: this & Object Prototypes

Note

This is a pretty bad example, since "class member properties" are actually static fields. As the name suggests it is kept in static memory. One more example of how instantiation isn't "copying."