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I keep having to change window.location.href depending on whether i'm developing on localhost or pushing to the server

I'm using google sign in and that's working. Once signed in I've got some code in app.js that opens a new webpage using:
window.location.href
But if I'm developing then the webpage is at:
localhost:3000/webpage.html
But then the URL changes when I push to the server.
Is there a way to use "or" so it will open on the localhost or on the server depending on where this is hosted?

The answer is: Use relative links.
Simply strip the host part from urls in your JavaScript file, like this:
From:
http://example.com/yourpage.htm
To:
/yourpage.htm
Now it will work, both locally and on your production server.
When you set 'location.href' to the relative url, the browser will add the current host name to the url.

Relative Paths
You might be asking about relative paths. A relative path doesn't contain the main domain, it only includes the path.
Instead of using:
localhost:3000/webpage.html
You could use: /webpage.html
Learn more about absolute vs. relative paths here
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Alternatively, if the question is about how to redirect to the public url (a .com address instead of localhost) when in production, take a look at this already answered question.

JQuery AJAX URL Local or Absolute

I have a portion of my web application that uses AJAX. I am using the JQuery AJAX call to add and remove items from the users shopping cart, and it works fine. The URL that I pass as a parameter in the AJAX call is the url-mapping defined in the web.xml file, minus the leading forward slash.
I tried to replicate this with another place in the application where I would like to use AJAX, but this time, the Servlet GET is not even being called at all. I have alerts in the javascript function, so I know the function is being called. Is there a way that I can see (within the ajax function call) what the fully qualified URL is that the GET request is being made to? Also, what is the 'rule' with respect to the AJAX URL? Is there a way that I can get to the context root? My Servlet is mapped to www.mydomain.com/contextRoot/un
I would rather not have to hardcode the context root into the function call, that way if it ever changes, I don't have to update all my javascript functions.

The "rule"
Ajax requests (or any http request from the page) that does not start with a / (or the full domain path) is relative to the current HTML file path.
How to view GET requests
Use the debugging tools in Chrome/Firefox or something like Fiddler2
How to get server root
If it is a simple www.host.com:
var root = location.protocol + '//' + location.host;
(Not sure if this works for context based domains).
You do not mention what server technology you are using, but if you are using MVC/Razor you can inject the server root into a Javascript variable with something like this in your master page header:
<script type="text/javascript>
window.domainRoot = "#(Url.Content("~/"))";
<script>
this injects a javaScript line like:
window.domainRoot = "http://www.mydomain/mycontext/";
you can then reference window.domainRoot from any JavaScript code.
If you are using PHP there will be an equivalent way to inject the server root.

A simple browser debugger (F12) and you can read the network traffic send.
From here it is possible to see all the details (sent and/or received)

I think The url plus parameter is contextRoot/un?para=*.

JQuery ajax .load() to reference to the correct url

I have a localhost with a few projects, so this project in VS2010 properties->web->use local IIS web server = 'localhost/project4/'. (I can't use the normal debug VS Development Server as some components won't work in my project) Anyhow, localhost is fine no big deal, so I go on coding.
Now, in ajax when I call .load('/Account/LogOn'); it gives me 'localhost/Account/LogOn' but what I really want is 'localhost/project4/Account/LogOn' because it is sitting in the project4 directory, not the root.
Any idea how to tell ajax I need that virtual directory prefix between domain name and the rest of the url?
EDIT --------------------------------------------
Thanks guys, combined with all your knowledge, I guess the best ways to do it are:
Include the js script into .cshtml server side and use "~/Account/LogOn/" let .net figure out the path.
Make a global var remove_me_debug_only_virtual_directory = "/project4/"; in js code. append it to the domain name. this way we don't have to pull .net into the water and write js code in .cshtml.
Move the project to localhost/ root if you can, in this case I can't, because other people at work wants to access this networked server and do demo.

If your JS code is in an MVC cshtml file, use it like this:
.load('#Url.Action("LogOn", "Account")');
The proper URL will be placed in the concrete JS code.

According to .net documents,
Absolute and relative path references in a server control have the following disadvantages:
Absolute paths are not portable between applications. If you move the application that the absolute path points to, the links will break.
Relative paths in the style of client elements can be difficult to maintain if you move resources or pages to different folders.
To overcome these disadvantages, ASP.NET includes the Web application root operator (~), which you can use when specifying a path in server controls. ASP.NET resolves the ~ operator to the root of the current application. You can use the ~ operator in conjunction with folders to specify a path that is based on the current root.
So as described above, using ~ will lead you to the root of your web project using asp:
<asp:image runat="server" id="Image1" ImageUrl="~/Images/SampleImage.jpg" />
For more information go to: Web Project Paths
Based on your update: Possible duplicate of: base url using jQuery
Where stated by #gregguida:
var l = window.location;
var base_url = l.protocol + "//" + l.host + "/" + l.pathname.split('/')[1];
//all you need to do is work with l.pathname.split('/') to find what you need.

I am a java developer but the context is same in both case,
In your can follow the steps
Check the context of your server if root is "/" then you have to call something like,
.load('/project4/Account/LogOn');
If your application is root and you can calling this request from itself then
.load('Account/LogOn');

asp.net MVC routing - javascript url 404 errors

While developing my app (asp.net mvc3) locally everything was fine using the VS dev app server. The app was located at localhost/. However, I'm attempting to deploy the application on a IIS 7.5 server in a /Management directory and having a lot of routing issues as a few calls in my app rely on the app being at the route.
I have some javascript code that calls my controller through an ajax call that looks like this:
url: "/en/Home/GetFormula/"
I would like it to go to: /Management/en/Home/GetFormula but instead it's going to the root of the site and looking for /en/Home/GetFormula and returning 404 errors.
Any ideas on how I can fix my javascript routing to default /Management/ as the root of the site?
Thanks

When the pseudo-URL passed to an HTTP request begins with a slash (e.g. "/path/to/resource"), the pseudo-URL is "completed" by treating it as if the given path were under the Web site's root directory (e.g. "http://my.site/path/to/directory").
Clearly, you were expecting the pseudo-URL to be processed as if the given path were under your Web application's root directory. Well, I have bad news for you: The HTTP protocol does not deal with such a thing as a "Web application".
The ASP.NET MVC Framework provides the Url.Content function, which takes pseudo-URLs beginning with a tilde character (e.g., "~/path/to/resource") and returns the result of replacing the tilde character with the Web application's root directory (e.g., "http://my.site/an/application/path/to/resource", assuming the Web application's root directory is "http:/my.site/an/application"). However, the ASP.NET MVC Framework is only available on the server side. If your JavaScript runs on the client side, it cannot call Url.Content.
But not all is lost. The ASP.NET MVC Framework allows you to dynamically generate JavaScript code on the server and run it on the client, the same way it allows you to dynamically generate HTML content and of course send it to the client. That way, you can expand the pseudo-URLs into actual URLs on the server side, and deploy the resulting JavaScript code to the client.

To avoid confusion about where you are currently try:
url: document.URL + "/Management/Home/GetFormula"

I solved this issue adding a html hidden field on my page where, on the server side, I put the correct url inferred with the Url.RouteUrl method like this:
<input id="MyHiddenFieldName" name="MyHiddenFieldName" type="hidden" value="#Url.RouteUrl(new { area = "MyArea", controller = "MyController", action = "MyAction" />
then, on your javascript code you could do this:
url: $("#MyHiddenFieldName).val()

Problem with forward slash "/" in jQuery AJAX

I always wrote URLs used by AJAX calls in this way: "/Home/Save" with the forward slash in the beginning. Now this last project is being deployed to a virtual directory on a server. Thus, these URLs aren't working anymore, because instead of "example.com/VirtualDir/Home/Save", they would point to "example.com/Home/Save" which is wrong. I quickly fixed the problem by removing the first forward slash "/" in all occurrences of URLs in my JavaScript. All pages work great, except for one! When AJAX call happens on the problematic page, the specified URL gets appended to the page URL. I've spent a few hours yesterday and the whole morning today, and I cannot figure it out. There is absolutely nothing different about this page comparing to others. Has anyone had this problem before? Should I post my code?
EDIT: After banging my head on the keyboard for another few hours, I ended up implementing the following. I got an action in a common Controller that returns the result of Request.Url.GetLeftPart(UriPartial.Authority), which is your http://www.mysite.com. I render it inside my Layout page into a global JavaScript variable, _AppPath. Then, every AJAX call gets its URL like this: _AppPath + '/Controller/Action'. This works everywhere and I still don't know what the hack is the problem with that page. Cheers!

Can you change the Ajax requests so that they instead point to "/VirtualDir/Home/Save"?
If it helps your code, you could have a path variable, so that you can easily update the virtual directory path (or remove it) when you deploy it somewhere else. Or your code could read its location via the window.location.href property and work out things out from there.
It's not so useful to have paths relative to the current document (i.e. without the / slash prefix) because, as you are observing, some of the pages will then fail their requests, when those pages are at a different point in the site hierarchy. An absolute URL would be the one to go for (i.e. with a / slash prefix).
[UPDATED, based on comments below]
#Dimskiy, it doesn't so much matter that the server-side framework is .NET MVC, or that there are no actual folders for those URLs on the server. The browser will just respond according to the URL structure it sees.
So the things to look for are the URLs in the browser address bar for the different pages, and the URLs of the Ajax requests being made to the server (e.g. look for these in Firebug's "Net" panel). And compare the URLs, looking at the number of folders suggested by each URL.
It doesn't matter if there isn't an actual folder on the server. The browser can't tell, it can only look at the URL structure. If the JavaScript is making a call from a page called "foo" to an Ajax resources at "Home/Save", then the request will be routed to "foo/Home/Save". And if the request is made from page "foo/bar" then it will be routed to "foo/bar/Home/Save". That's a relative path - it's relative to the containing HTML document.
A request to an "absolute" path, say, "/Home/Save" (note the / slash prefix) will always go to the root of the domain, e.g. example.com/Home/Save. But since you need your request to go to the "VirtualDir" virtual directory, then your URL will become "/VirtualDir/Home/Save".