Is there a way to access the page HTML source code using javascript?
I know that I can use document.body.innerHTML but it contains only the code inside the body. I want to get all the page source code including head and body tags with their content, and, if it's possible, also the html tag and the doctype. Is it possible?
Use
document.documentElement.outerHTML or
document.documentElement.innerHTML 
outerHTML.
This can be done in a one-liner using XMLSerializer.
var generatedSource = new XMLSerializer().serializeToString(document); Which gives String
<!DOCTYPE html><html><head> <title>html - javascript page source code - Stack Overflow</title> ... 
One way to do this would be to re-request the page using XMLHttpRequest, then you'll get the entire page verbatim from the web server.
For IE you can also use:
document.all[0].outerHTML 
Provided that
the page source can be re-downloaded:
fetch(document.location.href) .then(response => response.text()) .then(pageSource => /* ... */) 
fetch API is typically used by client-side JS to talk to REST endpoints, but using the fetch API on a non-REST endpoint doesn't magically turn it into a REST one. But even if we talk REST, statelessness is irrelevant. Two identical REST GET requests can return different data if the resource was actually modified between the requests, or your permission to access the resource was revoked, or for a number of other reasons.Accept header similar to that of the browser. But yeah, this approach is not generally reliable.