how to return data from php to jquery AJAX -

i'm new ajax, want learn. have set form (formidable pro) submitted via ajax. in submit button have on onclick function:

<div class="frm_submit"> <input id="btn-max-hyp" onclick="add_customer()" type="submit" value="[button_label]" [button_action] /> <img class="frm_ajax_loading" src="[frmurl]/images/ajax_loader.gif" alt="sending"/> </div>

my custom js file:

jquery(document).ready(function($){  $('#form_maxhyp').on("submit", add_customer);    function add_customer () {  var form_maxhyp = $(this).serialize();    $.ajax({  type:"post",  url: frontendajax.ajaxurl,  data: form_maxhyp,    success: function(data)  {  	$("#test").html(data);  }  });    return false;    }  });

and last code in function.php

<?php  add_action( 'wp_enqueue_scripts', 'add_frontend_ajax_javascript_file' );  function add_frontend_ajax_javascript_file()  {  	wp_enqueue_script( 'ajax_custom_script',  get_stylesheet_directory_uri() . '/ajax-javascript.js', array('jquery') );  	wp_localize_script( 'ajax_custom_script', 'frontendajax', array( 'ajaxurl' => admin_url( 'admin-ajax.php' )));  }    add_action('wp_ajax_add_customer', 'add_customer');  add_action('wp_ajax_nopriv_add_customer', 'add_customer');  function add_customer($entry_id, $form_id)  {  	if ( $form_id == 6 )   	{  		global $wpdb, $frmdb;  		$form1 = '6';          $value = $_post['item_meta'][222];    /* lot of if , else statements */    /* echo values */  		echo "'".$value. "'";  	}  	  	die();  }

i want set value div. in console log error: referenceerror: add_customer not defined.

probably it's simple i'm not seeing. i'm doing wrong here?

in custom js file have :

jquery(document).ready(function($){ $('#form_maxhyp').on("submit", add_customer);

in case add_customer read variable.

change this: jquery(document).ready(function($){ $('#form_maxhyp').on("submit", add_customer());

to make call function!