codeigniter form using ajax jquery and json encode - php

Question

I am trying to make a contact form, using ajax and jquery, I am returning the validation errors using json encode, when I try to display the errors as an alert box, all it says in the alert box is undefined.
form
<?php echo form_open('page/contact_validation',array('class'=> 'contact_form'));?>
<div class="row">
<div class="large-6 columns large-centered">
<div class="row">
<div class="medium-12 columns">
<label>Name
<input type="text"placeholder="Enter Name" name="name"id="name">
</label>
<div class="jsError_name"></div>
</div><!--medium 12-->
</div><!--row-->
<div class="row">
<div class="medium-12 columns">
<label>Email
<input type="text"placeholder="Enter Email"name="email" id="email">
</label>
<div class="jsError_email"></div>
</div><!--medium 12-->
</div><!--row-->
<div class="row">
<div class="medium-12 columns">
<label>Phone Number
<input type="text"placeholder="Enter Number">
</label>
</div><!--medium 12-->
</div><!--row-->
<div class="row">
<div class="medium-12 columns">
<label>Message
<input type="text"placeholder="Message">
</label>
</div><!--medium 12-->
</div><!--row-->
<div class="row column">
<button type="submit" class="button" value="Send"id="submit">
<span aria-hidden="true"><i class="fi-mail"></i></span>
Send</button>
<button class="button reset"type="reset"value="reset">Reset</button>
</div>
</div>
</div><!--row-->
<?php echo form_close();?>
jquery
$(document).ready(function(){
$('form.contact_form').on('submit',function(e){
e.preventDefault();
$.post('contact_validation', $('form.contact_form').serialize(), function(data) {
alert(data.firstvalue);
});
});
});
contact validation function
public function contact_validation() {
$formRules =array(
array(
'field' =>'name',
'rules' => 'required|min_length[5]|max_length[30]'
),
array(
'field' =>'email',
'rules' => 'required|min_length[5]|max_length[30]'
)
);
$this->form_validation->set_rules($formRules);
if($this->form_validation->run()== true) {
echo 'success';
} else {
echo json_encode(array('firstvalue' => form_error('name'),'secondvalue' => form_error('email')));
}
}
}