Structures.extend doesn't search the prototype chain

[Dragoteryx]

Is your feature request related to a problem? Please describe.
I started to work on my own Discord.js framework, and I created a mix-in called CommandManager.

// my CommandManager mixin function, it takes a superclass as a parameter and returns a new class that inherits properties from that superclass
CommandManager = superclass => class extends superclass {
  constructor(...args) {
    super(...args);
  }
  createCommand(init, run, options) {
    let command = new Command(this, init, run, options);
    prv(this).commands.set(command.name, command);
    return command;
  }
}

So this is what my Guild extender looks like:

module.exports = Structures.extend("Guild", Guild => {
  class GungnirGuild extends CommandManager(Guild) {
    constructor(...args) {
      super(...args);
    }
  }
  return GungnirGuild;
});

Simple enough, this creates a new class called GungnirGuild, which inherits properties from CommandManager, which inherits properties from Guild. Except the check inside the extend function throws an Error, because it checks for the prototype of the returned class and doesn't check its prototype chain.

Describe the ideal solution
Use instanceof instead of directly comparing the prototypes.
Replace this:

const prototype = Object.getPrototypeOf(extended);
    if (prototype !== structures[structure]) {
      const received = `${extended.name || 'unnamed'}${prototype.name ? ` extends ${prototype.name}` : ''}`;
      throw new Error(
        'The class/prototype returned from the extender function must extend the existing structure class/prototype' +
        ` (received function ${received}; expected extension of ${structures[structure].name}).`
      );
    }

with this:

if (!(extended.prototype instanceof structures[structure])) {
      const prototype = Object.getPrototypeOf(extended);
      const received = `${extended.name || 'unnamed'}${prototype.name ? ` extends ${prototype.name}` : ''}`;
      throw new Error(
        'The class/prototype returned from the extender function must extend the existing structure class/prototype' +
        ` (received function ${received}; expected extension of ${structures[structure].name}).`
      );
    }

This will check if the extended class is in the prototype chain, and will work even if the returned class doesn't directly inherit from the extended class.

Describe alternatives you've considered
Defining my command related functions/properties on every class I want them to be available, but that would take a while and is an anti-pattern.

[Dragoteryx]

Another example that doesn't use mixins:

module.exports = Structures.extend("Guild", Guild => {
  class CoolGuild extends Guild {}
  class CoolerGuild extends CoolGuild {}
  return CoolerGuild;
});

Even though CoolerGuild extends Guild (indirectly), Structures.extend will return an error. Using instanceof fixes this.

[bdistin]

I don't see a problem with your proposed solution, however, I am confused on why this is an issue instead of a pr since you are proposing a solution?

[Dragoteryx]

Oh yeah, you're right. Just realised that lol. Going to make a push request.