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HTML 

              
                <h3>Combined Login/Registration Form</h3>
<p>If usernames are not public, DO NOT DO THIS.</p>

<p>There are two things going on here.</p>
<ol>
  <li>Typing in a username that already exists gives an immediate reaction, changing the form. Only one form is necessary!</li>
  <li>When registering, you are only alerted of passwords not being equal if they are of the same length. (why is this not a thing?)</li>
</ol>

<p>In our imaginary database, "ShuckleFan3" is a user.</p>


<div id="formMessage" class="message"></div>

<form>
  <label>
    							Username (16 chars max)
							    <input id="login" type="text" name="login" placeholder="username (16 chars max)">
						  </label>
						  <label>
							    Password (6 chars +)
    							<input id="password" type="password" name="password" placeholder="password (6 chars +)">
  						</label>
						  <label>
							    Registering: Password again
							    <input id="password2" type="password" name="password2" placeholder="registering: password again">
							    <p id="userExists" style="display:none;">User Exists</p>
						  </label>
						  <button id="logRegButton">Login / Register</button>
</form>


<p>See a live example using this at http://www.story-ga.me/</p>
!

CSS 

              
                label {
  display:inline-block;
  margin:0 1em 0 0;
  padding:.5em;
  background-color:#e0e0e0;
}

input, #userExists {
  display:block;
  	margin:0;
  	padding:.1em;
	  width:13em;
	  height:1.4em;
	  line-height:1.4em;
	  font-size:1em;
  border:1px solid #666;
  font-family:arial;
}
#userExists {
	  background-color:#3dff94;
}

.message {
  width:13em;
  height:2em;
  line-height:2em;
  padding:0 1em;
  margin-bottom:1em;
  background-color:lightblue;
  color:darkblue;
}
!

JS 

              
                window.isRegistering = true;

function checkUsers(username)
{
  /* Checking the database for pretend right here. */
  if (username == "ShuckleFan3")
  {
    /* User Exists! No registration needed. */
    document.getElementById("password2").value = "";
			    document.getElementById("password2").style.display = "none";
			    document.getElementById("userExists").style.display = "block";
    window.isRegistering = false;
  }
  else
  {
    document.getElementById("userExists").style.display = "none";
    			document.getElementById("password2").style.display = "block";
    window.isRegistering = true;
  }
}

function checkPass(pass1, pass2)
{
  /* Not going to say anything if the user isn't finished typing. */
  if (pass1.length == pass2.length && window.isRegistering)
  {
    if (pass1 !== pass2)
      formMessage("The passwords are not the same.");
    else
      formMessage("");
  }
  else
    formMessage("");
}

function formMessage(message)
{
  			document.getElementById("formMessage").innerHTML = message;
}


/* Whenever a key is entered in the login field, we check (via ajax, etc) if that user already exists. */
document.getElementById("login").onkeyup=function(e){
  checkUsers(document.getElementById("login").value);
}
/* Whenever a key is entered in the password fields, we check (via ajax, etc if the passwords are the same) */
document.getElementById("password").onkeyup=function(e){
  checkPass(document.getElementById("password").value, document.getElementById("password2").value);
}
document.getElementById("password2").onkeyup=function(e){
  checkPass(document.getElementById("password").value, document.getElementById("password2").value);
}
/* You'd want to rate-limit these requests, of course. */
!
999px

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