Untitled

<html>
<body>

<textarea id="emailTextarea" rows ="30" cols="30" >
sara@yahoo.com
adam@yahoo.com
todd@yahoo.com
henry@yahoo.com
wright@yahoo.com
</textarea>

<p id="demo"></p>

<script>

//The email list
    var emailList = document.getElementById("emailTextarea").value;

//remove the extension
    var emailUserHash = emailList.reduce(function(emailUsers, email, i) {
        var username = email.slice(0, email.indexOf('yahoo.com'));
        if(!emailUsers[username]) emailUsers[username] = true;
        return emailUsers;
    }, {});

//call the emailUserHash function
    var emailUsers = Object.keys(emailUserHash)

//sort the email list
    emailUsers.sort();

//output the list
    document.write(emailUsers.join('</br>'));
    </script>
</body>