Search code examples
javascriptiframeinstanceof

Instanceof fails in iframe

The following code returns true.

console.log(document.createElement('script') instanceof Element);

Doing the same in an <iframe> context returns false:

let iframe = document.querySelector('iframe');
iframe = iframe.contentDocument || iframe.contentWindow.document;

console.log(iframe.createElement('script') instanceof Element);

Demo

Why is that?

Solution

  • This is because:

    1) Element is actually window.Element

    2) In JS there is no such thing as a "class". Everything (almost) is an object. So instanceof checks for Prototype ancestry. When you ask is some DOM node instanceof Element you could kind of translate this to someDOMNode.prototype === Element.

    3) window.Element !== document.querySelector('iframe').contentWindow.Element !!!

    This will log true as expected:

    console.log(iframe.createElement('script') instanceof  document.querySelector('iframe').contentWindow.Element);