wp-ajax.txt

1. In functions.php file
=========================

if(condition){
  wp_enqueue_script( 'handler', 'path', array(), 'version', boolean );
  
  $ajax_url = admin_url('admin-ajax.php');
  wp_localize_script('handler', 'object_name', array('data_1', => 'value_1') );
}

3. Hook data to built ajax.php
==============================
function callback_fucntion() {

	if ( check_ajax_referer( 'action_name', 'nonce_value' ) ) {
		$info = $_POST['info'];
		echo strtoupper( $info );
		die(); // Important
	}
}
add_action( 'wp_ajax_{action_name}', 'callback_fucntion' ); // loged in user
add_action( 'wp_ajax_nopriv_{action_name}', 'callback_fucntion' ); // non loged in user ( We can define another function for the non logged in users )

2. In html file
====================
wp_nonce_field('ajaxtest');

3. In ajax.js file
====================

console.log(object_name.data_1); // retrieve ajax url

var info = $("#info").val();
  var nonce = $("#_wpnonce").val();

  $.post(object_name.data_1, {
    action: "action_name", // important to keep in mind
    info: info,
    nonce_value:nonce
  }, function (data) {
    console.log(data);
  });