How to construct a WebSocket URI relative to the page URI?

Here is my version which adds the tcp port in case it's not 80 or 443:

function url(s) {
    var l = window.location;
    return ((l.protocol === "https:") ? "wss://" : "ws://") + l.hostname + (((l.port != 80) && (l.port != 443)) ? ":" + l.port : "") + l.pathname + s;
}

Edit 1: Improved version as by suggestion of @kanaka :

function url(s) {
    var l = window.location;
    return ((l.protocol === "https:") ? "wss://" : "ws://") + l.host + l.pathname + s;
}

Edit 2: Nowadays I create the WebSocket this:

var s = new WebSocket(((window.location.protocol === "https:") ? "wss://" : "ws://") + window.location.host + "/ws");

If your Web server has support for WebSockets (or a WebSocket handler module) then you can use the same host and port and just change the scheme like you are showing. There are many options for running a Web server and Websocket server/module together.

I would suggest that you look at the individual pieces of the window.location global and join them back together instead of doing blind string substitution.

var loc = window.location, new_uri;
if (loc.protocol === "https:") {
    new_uri = "wss:";
} else {
    new_uri = "ws:";
}
new_uri += "//" + loc.host;
new_uri += loc.pathname + "/to/ws";

Note that some web servers (i.e. Jetty based ones) currently use the path (rather than the upgrade header) to determine whether a specific request should be passed on to the WebSocket handler. So you may be limited in whether you can transform the path in the way you want.

Using the Window.URL API - https://developer.mozilla.org/en-US/docs/Web/API/Window/URL

Works with http(s), ports etc.

var url = new URL('/path/to/websocket', window.location.href);

url.protocol = url.protocol.replace('http', 'ws');

url.href // => ws://www.example.com:9999/path/to/websocket