Selenium Javascript - Access webelements within iframe programatically

Question

I am trying to access web elements within iFrame programatically. Using below method to check the same

• Compare the URL of elements with the base URI
• If its same then web element is in main DOM otherwise its in iFrame
• Use switchTo.frame() to switch to iFrame and perform actions

In above method, the following methods used to get URI of element

document.getElementsByTagName("a")[0].baseURI;

or

document.getElementsByClassName("w3-btn w3-border")[0].baseURI

Queries:

1. Is there any function or method to get baseURI using xpath? I don't want to use Tagname or classname.

2. Also, how to achieve same when multiple iFrames or iFrame inside another iFrame.

3. Kindly suggest if any other method