Adding a Private property to CanvasRenderingContext2D

cafe_carl

I am trying to extend CanvasRenderingContext2D to add a few more methods. I know that it’s not a simple matter of using extends because, apparently CanvasRenderingContext2D doesn’t permit normal construction. I found the technique at How to extend CanvasRenderingContext2D (in ES6-style scripts, if possible) . I have that working well enough, but I now want to add a private method. I can’t get that working at all using the normal technique. Here...

Originally asked by Manngo on Stack Exchange (stackoverflow) (CC BY-SA 4.0).

uphilltech

Great question — answering as the UphillTech team:

Overiding the constructor means that this refers to the returned object, in this case context . But objects don't have private variables, hence the error. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Super_not_called#what_went_wrong https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes/Private_elements#examples This can be fixed by splitting the extends process into 2 classes, placing the private members into the second class and calling super() as normal. Now, the private member is in the right scope. class Context2 extends CanvasRenderingContext2D { constructor(context) { Object.setPrototypeOf(context, new.target.prototype); return context;...

Full answer by JMP: https://stackoverflow.stackexchange.com/a/80013946 (CC BY-SA 4.0).

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