In javascript, can I assign iframe doc to document variable?

I have a Java Script code that works inside Iframe. I’d like to save the Iframe context so that my next script can work in the same context.

The following is my code

// Get the iframe element
const iframe = document.evaluate("/html[1]/body[1]/iframe[1]", document, null, XPathResult.FIRST_ORDERED_NODE_TYPE, null).singleNodeValue; 

// Access the iframe's document object
const iframeDoc = iframe.contentDocument || iframe.contentWindow.document;

// use iframe doc directly
var e1 = iframeDoc.evaluate("id('shadow_host')", iframeDoc, null, XPathResult.FIRST_ORDERED_NODE_TYPE, null).singleNodeValue;
console.log(`e1=${e1}`);

// can I assign iframeDoc to document?
document = iframeDoc;
var e2 = document.evaluate("id('shadow_host')", document, null, XPathResult.FIRST_ORDERED_NODE_TYPE, null).singleNodeValue;
console.log(`e2=${e2}`);

I got the following result

e1=[object HTMLDivElement]
e2=null

Looks like if I use Iframe doc directly, I able able to search inside the Iframe context. I did this with e1.

But If I assign the Iframe doc to ‘document” variable, I am not able to search inside the Iframe context. I did this with e2.

Why my e2 example doesn’t work?

I really wanted to use ‘document’ variable because it is global and I don’t have to pass a variable to my next call.

Thank you for any help