I loaded a website of my own design in an iframe. This website contains links to other cross-domain pages. I wrote a javascript function to retrieve the title of the current page contained in the iframe, but when I run my application at localhost:3000, I get the error:

Uncaught DOMException: Blocked a frame with origin "http://localhost:3000" from accessing a cross-origin frame.

Even when I am using this javascript method on the page that was originally loaded in the iframe, I still get the same error.
I would like to know exactly what is causing this error.