Class

An object created from a class will always be as an instanceof its class.

class classCat { 
  constructor(name) {
    this.name = name;
   }
}

var myClassCat = new classCat("Poesie");
console.log(myClassCat instanceof classCat); // true

Function

• The constructor of a function created WITHOUT a prototype will be NOT recognized as a instanceof

var Cat = function(name) {
    obj = {};
    obj.name = name;
    return obj;
}

var myCat = Cat("Poesie");
console.log(myCat instanceof Cat); // false

• The constructor of a function created WITH a prototype will be recognized as a instanceof

var Cat = function(name) {
    var obj = Object.create(Cat.prototype);
    obj.name = name;
    return obj;
}
Cat.prototype = {};

var myCat = Cat("Poesie");
console.log(myCat instanceof Cat); // true

DOM Element and inheritance

instanceof works also with inheritance.

<div>A DivElement</div>

let element  = document.querySelector("div");
console.log(element instanceof HTMLDivElement);
console.log(element instanceof HTMLElement);
console.log(element instanceof Element);

Iframe: Why false ?

Because an iframe creates a new browsing context.

Try this ?

myVariable instanceof mtVariable.ownerDocument.defaultView.Element