How to get the host URL using JavaScript from the current page

// will return the host name and port
var host = window.location.host; 

or possibly

var host = window.location.protocol + "//" + window.location.host;

or if you like concatenation

var protocol = location.protocol;
var slashes = protocol.concat("//");
var host = slashes.concat(window.location.host);

// or as you probably should do
var host = location.protocol.concat("//").concat(window.location.host);

// the above is the same as origin, e.g. "https://stackoverflow.com"
var host = window.location.origin;

If you have or expect custom ports use window.location.host instead of window.location.hostname