Check if object is a 'direct instance' of a class

Question

I have two classes:

class Bar extends Foo { // Foo isn't relevant
  constructor(value) {
    if (!(value instanceof Foo)) throw "InvalidArgumentException: (...)";
    super();
    this.value = value;
  }
}

class Baz extends Bar {
  constructor(value) {
    super(value);
  }
}

The Bar constructor checks if value is an instance of Foo, it throws an error if it isn't. At least, that's what I wanted it to do. If you pass a Bar or a Baz as value, the if-statement returns true as well. The goal is to only let Foos through.
I found this answer already but that didn't really answer my question.

Answer 1

Check the constructor:

if (!value || value.constructor !== Foo)
  throw 'InvalidArgumentException: (...)';

or the prototype of the object (this is more similar to what instanceof does):

if (!value || Object.getPrototypeOf(value) !== Foo.prototype)
  throw 'InvalidArgumentException: (...)';

Answer 2

You can use a comparison between Object.getPrototypeOf(yourObj) and Foo.prototype to see if yourObj is exactly an instance of Foo. And you can move up the chain by just continuing to call Object.getPrototypeOf for each level.

Example:

class Foo {}

class Bar extends Foo {}
class Baz extends Bar {}

const foo = new Foo();
const bar = new Bar();
const baz = new Baz();

// For this function:
// - level 0 is self
// - level 1 is parent
// - level 2 is grandparent
// and so on.
function getPrototypeAt(level, obj) {
    let proto = Object.getPrototypeOf(obj);
    while (level--) proto = Object.getPrototypeOf(proto);
    return proto;
}

console.log("bar is a foo:", bar instanceof Foo);
console.log("baz is a foo:", baz instanceof Foo);
console.log("foo is exactly a foo:", getPrototypeAt(0, foo) === Foo.prototype);
console.log("bar is exactly a foo:", getPrototypeAt(0, bar) === Foo.prototype);
console.log("bar is direct child of foo:", getPrototypeAt(1, bar) === Foo.prototype);
console.log("baz is direct child of foo:", getPrototypeAt(1, baz) === Foo.prototype);
console.log("baz is direct child of bar:", getPrototypeAt(1, baz) === Bar.prototype);
console.log("baz is grandchild of foo:", getPrototypeAt(2, baz) === Foo.prototype);