How to go from one page to another page using javascript?

Try this,

window.location.href="sample.html";

Here sample.html is a next page. It will go to the next page.

To simply redirect a browser using javascript:

window.location.href = "http://example.com/new_url";

To redirect AND submit a form (i.e. login details), requires no javascript:

<form action="/new_url" method="POST">
   <input name="username">
   <input type="password" name="password">
   <button type="submit">Submit</button>
</form>

You cannot sanely depend on client side JavaScript to determine if user credentials are correct. The browser (and all code that executes that) is under the control of the user, not you, so it is not trustworthy.

The username and password need to be entered using a form. The OK button will be a submit button. The action attribute must point to a URL which will be handled by a program that checks the credentials.

This program could be written in JavaScript, but how you go about that would depend on which server side JavaScript engine you were using. Note that SSJS is not a mainstream technology so if you really want to use it, you would have to use specialised hosting or admin your own server.

(Half a decade later and SSJS is much more common thanks to Node.js, it is still fairly specialised though).

If you want to redirect afterwards, then the program needs to emit an HTTP Location header.

Note that you need to check the credentials are OK (usually by storing a token, which isn't the actual password, in a cookie) before outputting any private page. Otherwise anyone could get to the private pages by knowing the URL (and thus bypassing the login system).