Password protect html pages, 1 time login

I have a question about html/JavaScript/PHP, I want to be able to view a few pages only if I fill in the right password, but I don't want that you have to put the password in on every private page you visit.

For example:

pages:

• Login.html
• PrivatePage1.html
• PrivatePage2.html

Now you can just go to: 'http://ift.tt/2uAb4bd' and you don't have to give a password, so is there a way to check if they already logged in on 'Login.hmtl'

My code on 'Login.html'

http://ift.tt/2w0mA3b

password();

function password() {
    var userPassword;
    var myPassword = "a";

    userPassword = prompt("Enter the password:");

    if(userPassword == myPasswordfun) {
        alert('Correct, press ok to enter the site.');
    }else if(userPassword == null) {
        window.location ="http://www.google.com";
    }else {
        alert('Incorrect');
        password();
    }
}

Thanks for reading!