"Class extends value #<Object> is not a constructor or null"

When I get this error message it is because I have done my module.exports wrong. eg.

publicclass.js

class PublicClass {
.....
}

module.exports.PublicClass = PublicClass;

instead of

module.exports = PublicClass;

I see at least one issue with your requires.

• monster.js first line is const miniMonster = require("./minimonster.js");
• minimonster.js first line is const monster = require("./monster.js");

This is a problem, you can not have both files evaluate at the same time. I would not require minimonster from monster.js

This may fix your issue.