How can I use JavaScript to view the source code of all websites?

Question

I want to have JavaScript to get a user's URL and return the source code of the website. I want to have this to essentially make an iframe, but with the actual code.

E.g. :

let userUrl = prompt("What website do you want displayed?","e.g. https://www.google.com");

function getSource(url){

//some code
return pageSource;
}
document.body.innerHtml=getSource(userUrl);

I tried to scrape a view page source website and turn it into an API that I could inject into JavaScript, but I had no luck.