Pure JavaScript code for HTTP Basic Authentication?
The five-parameter version of the XMLHttpRequest.open method allows you to specify the username and password. (WHATWG spec)
xhr.open(method, url, async, username, password)
There is a good article/tutorial written by Paul James. I used it some time ago and it worked for me.
HTTP Authentication with HTML Forms
[...] XMLHTTPRequest, it can submit the correct HTTP auth headers for us. Rather than adjusting the URL the form submits to, we can use XMLHTTPRequest to do a request before the form submits supplying the entered username and password.
This will set up the browser with the HTTP auth credentials so it'll also send them with our actual form submission login request.