function post(){
        $.post("<?php echo base_url();?>index.php/user/create", {username:"testUser",password:"userpassword"},function(data) {
                var test = data;
        });
    }

public function create(){
        if($this->input->post('newuser')){
            $username = $this->input->post('username');
            $password = $this->input->post('password');

            echo json_encode(array('username'=>$username));
}

$.post("<?php echo base_url();?>index.php/user/create", {newuser: 1, username:"testUser",password:"userpassword"}...

<script type="text/javascript">
 function post(){
  var url ="<?php echo base_url();?>index.php/user/create";
   $.post(url, {username:"testUser",password:"userpassword"},function(data) {
      console.log(data);
      var test = data;
   });
 }
</script>

public function create(){
  if($this->input->post('username')){
    $username = $this->input->post('username');
    $password = $this->input->post('password');

    echo json_encode(array('username'=>$username));
  }
}