using instanceof with class returned from a function

Question

Since es6 doesn't support nested classes, I've taken to adding a nested class to a static getter of the parent. For example:

class Dictionary {
  static get Category () {
    return class Category {
      static is (inst) {
        return inst instanceof Category
      }
      constructor () {}
    }
  }
}

However, I'm having a problem with instanceof. I want to make sure a given variable is a correct instance.

const test = new Dictionary.Category()
const ctor = Dictionary.Category

console.log(test instanceof Dictionary.Category) // false
console.log(Dictionary.Category.is(test)) // false
console.log(test instanceof ctor) // false
console.log(test.__proto__.constructor === ctor) // false
console.log(test.constructor === ctor) // false
console.log(test.constructor, ctor) // both function Category()
console.log(new Dictionary() instanceof Dictionary) // true (sanity check)

Up till now, I've been testing the constructor.name but this is limited. How can I solve this problem?

Thanks to the answer below pointing out the problem, here is a working solution:

class Dictionary {
  static get Category () {
    Dictionary.__Category = Dictionary.__Category || class Category {
      constructor () {}
    }
    return Dictionary.__Category
  }
}

loganfsmyth · Accepted Answer · 2020-03-18 04:49:20Z

2

This is failing because every access of the getter property creates a whole new class, it just happens to have all the same stuff as the one you got last time you accessed the getter.

If you want a nested property, use a property, e.g.

class Dictionary {
}
Dictionary.Category = class Category {
  static is (inst) {
    return inst instanceof Category
  }
  constructor () {}
};

answered Mar 18, 2020 at 4:49

loganfsmyth

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3 Comments

Twifty Over a year ago

That makes sense. I was trying to avoid declaring the property outside of the class due to cluttering a GreaseMonkey script. In Node I would external files. Saying that though, I could add the internal class to window and have the getter return it.

loganfsmyth Over a year ago

I don't think I understand what you mean by clutter in this context. Why would a getter be more desirable than a property? If you're injecting a script into a page you can always use an IIFE to avoid polluting the global scope while still declaring all your classes next to eachother without nesting.

Twifty Over a year ago

By clutter, I mean I have a script currently at 6000 lines consisting of about 10 main classes. By having nested classes, I can fold the code in the editor reducing the visible lines to about 40. Also having nested classes makes them easier to find.

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